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+# Glossary
+
+The Tor Project
+
+This document aims to specify terms, notations, and phrases related
+to Tor, as used in the Tor specification documents and other documentation.
+
+This glossary is not a design document; it is only a reference.
+
+This glossary is a work-in-progress; double-check its definitions before
+citing them authoritatively. ;)
+
+<a id="glossary.txt-0"></a>
+
+## Preliminaries
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
+NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
+"OPTIONAL" in this document are to be interpreted as described in
+RFC 2119.
+
+<a id="glossary.txt-1.0"></a>
+
+## Commonly used Tor configuration terms { #configuration }
+
+ORPort  - Onion Router Port
+DirPort - Directory Port
+
+<a id="glossary.txt-2.0"></a>
+
+## Tor network components { #network-compoennts }
+
+<a id="glossary.txt-2.1"></a>
+
+## Relays, aka OR (onion router) { #relay}
+
+\[Style guide: prefer the term "Relay"\]
+
+<a id="glossary.txt-2.1.1"></a>
+
+### Specific roles { #roles }
+
+Exit relay: The final hop in an exit circuit before traffic leaves
+the Tor network to connect to external servers.
+
+Non-exit relay: Relays that send and receive traffic only to
+other Tor relays and Tor clients.
+
+Entry relay: The first hop in a Tor circuit. Can be either a guard
+relay or a bridge, depending on the client's configuration.
+
+Guard relay: A relay that a client uses as its entry for a longer
+period of time.  Guard relays are rotated more slowly to prevent
+attacks that can come from being exposed to too many guards.
+
+Bridge: A relay intentionally not listed in the public Tor
+consensus, with the purpose of circumventing entities (such as
+governments or ISPs) seeking to block clients from using Tor.
+Currently, bridges are used only as entry relays.
+
+Directory cache: A relay that downloads cached directory information
+from the directory authorities and serves it to clients on demand.
+Any relay will act as a directory cache, if its bandwidth is high enough.
+
+Rendezvous point: A relay connecting a client to a hidden service.
+Each party builds a three-hop circuit, meeting at the
+rendezvous point.
+
+<a id="glossary.txt-2.2"></a>
+
+## Client, aka OP (onion proxy) { #client }
+
+\[Style: the "OP" and "onion proxy" terms are deprecated.\]
+
+<a id="glossary.txt-2.3"></a>
+
+## Authorities { #authorities }
+
+Directory Authority: Nine total in the Tor network, operated by
+trusted individuals. Directory authorities define and serve the
+consensus document, defining the "state of the network." This document
+contains a "router status" section for every relay currently
+in the network. Directory authorities also serve router descriptors,
+extra info documents, microdescriptors, and the microdescriptor consensus.
+
+Bridge Authority: One total. Similar in responsibility to directory
+authorities, but for bridges.
+
+Fallback directory mirror: One of a list of directory caches distributed
+with the Tor software. (When a client first connects to the network, and
+has no directory information, it asks a fallback directory. From then on,
+the client can ask any directory cache that's listed in the directory
+information it has.)
+
+<a id="glossary.txt-2.4"></a>
+
+## Hidden Service { #hidden-service }
+
+A hidden service is a server that will only accept incoming
+connections via the hidden service protocol. Connection
+initiators will not be able to learn the IP address of the hidden
+service, allowing the hidden service to receive incoming connections,
+serve content, etc, while preserving its location anonymity.
+
+<a id="glossary.txt-2.5"></a>
+
+## Circuit
+
+An established path through the network, where cryptographic keys
+are negotiated using the ntor protocol or TAP (Tor Authentication
+Protocol (deprecated)) with each hop. Circuits can differ in length
+depending on their purpose. See also Leaky Pipe Topology.
+
+Origin Circuit -
+
+Exit Circuit: A circuit which connects clients to destinations
+outside the Tor network. For example, if a client wanted to visit
+duckduckgo.com, this connection would require an exit circuit.
+
+Internal Circuit: A circuit whose traffic never leaves the Tor
+network. For example, a client could connect to a hidden service via
+an internal circuit.
+
+<a id="glossary.txt-2.6"></a>
+
+## Edge connection
+
+```text
+2.7. Consensus: The state of the Tor network, published every hour,
+     decided by a vote from the network's directory authorities. Clients
+     fetch the consensus from directory authorities, fallback
+     directories, or directory caches.
+
+2.8. Descriptor: Each descriptor represents information about one
+    relay in the Tor network. The descriptor includes the relay's IP
+    address, public keys, and other data. Relays send
+    descriptors to directory authorities, who vote and publish a
+    summary of them in the network consensus.
+```
+
+<a id="glossary.txt-3.0"></a>
+
+## Tor network protocols
+
+<a id="glossary.txt-3.1"></a>
+
+## Messages and cells
+
+Cell: A message sent over a channel. Every cell has an associated
+command.  A cell may be fixed-length or variable-length, depending on
+its command.  Cells are sometimes referred to by their command types:
+for example, a cell whose command is `DESTROY` is called a
+DESTROY cell.
+
+Relay cell: A cell that tells a relay or client about instructions sent
+over a circuit.  The command of a relay cell may be `RELAY` or `RELAY_EARLY`.
+If we need to refer to a cell whose command is specifically `RELAY`,
+we call it a "RELAY" cell.
+
+Enveloped relay message: The results of decrypting a relay cell:
+a relay message plus an associated (optional) StreamID.
+(If the StreamID is not present, or zero,
+then the relay message is addressed to the circuit itself
+rather than to any particular stream on the circuit.)
+
+Relay message: A message sent over a circuit to an individual stream,
+or to the circuit itself. Relay messages are sometimes referred to
+by their command types: for example, a message whose command is
+`DATA` is sometimes called a DATA message.
+Sometimes, relay messages are just called "Messages"
+if no ambiguity would result.
+
+> Note that when [prop340](proposals/340-packed-and-fragmented.md) is implemented,
+> the relationship between relay cells and (enveloped) relay messages
+> will no longer be 1:1.
+
+## Link handshake
+
+The link handshake establishes the TLS connection over which two
+Tor participants will send Tor cells.  This handshake also
+authenticates the participants to each other, possibly using Tor
+cells.
+
+<a id="glossary.txt-3.2"></a>
+
+## Circuit handshake
+
+Circuit handshakes establish the hop-by-hop onion encryption
+that clients use to tunnel their application traffic.  The
+client does a pairwise key establishment handshake with each
+individual relay in the circuit.  For every hop except the
+first, these handshakes tunnel through existing hops in the
+circuit.  Each cell type in this protocol also has a newer
+version (with a "2" suffix), e.g., CREATE2.
+
+CREATE cell: First part of a handshake, sent by the initiator.
+
+CREATED cell: Second part of a handshake, sent by the responder.
+
+EXTEND message: (also known as a RELAY_EXTEND message) First part of a
+handshake, tunneled through an existing circuit.  The last relay
+in the circuit so far will process this message by
+decoding it,
+and sending the appropriate handshake
+in a CREATED cell to the client's chosen next-hop relay.
+
+EXTENDED cell: (also known as a RELAY_EXTENDED message) Second part
+of a handshake, tunneled through an existing circuit.  The last
+relay in the circuit so far receives the CREATED cell from the
+new last-hop relay,
+encodes that cell's body in in an EXTENDED message,
+and uses a RELAY cell to deliver the message back to the client.
+Upon receiving the EXTENDED message,
+the client's circuit is one hop longer.
+
+Onion skin: The body of a CREATE/CREATE2 cell or an EXTEND/EXTEND2 message.
+It contains the first part of the TAP or ntor key establishment
+handshake.
+
+<a id="glossary.txt-3.3"></a>
+
+## Hidden Service Protocol
+
+<a id="glossary.txt-3.4"></a>
+
+## Directory Protocol
+
+<a id="glossary.txt-4.0"></a>
+
+## General network definitions
+
+Leaky Pipe Topology: The ability for the origin of a circuit to address
+relay cells to be addressed to any hop in the path of a circuit. In Tor,
+the destination hop is determined by using the 'recognized' field of relay
+cells.
+
+Stream: A single application-level connection or request, multiplexed over
+a Tor circuit.  A 'Stream' can currently carry the contents of a TCP
+connection, a DNS request, or a Tor directory request.
+
+Channel: A pairwise connection between two Tor relays, or between a
+client and a relay. Circuits are multiplexed over Channels. All
+channels are currently implemented as TLS connections.