diff --git a/docs/specification.rst b/docs/specification.rst
index c5b6fe29c8..d331f36b4b 100644
--- a/docs/specification.rst
+++ b/docs/specification.rst
@@ -1809,12 +1809,76 @@ Glossary
 .. NOTE::
   This section is a work in progress.
 
-.. TODO
-  - domain specific words/acronyms with definitions
+Backfilling:
+  The process of synchronising historic state from one home server to another,
+  to backfill the event storage so that scrollback can be presented to the
+  client(s). Not to be confused with pagination.
+
+Context:
+  A single human-level entity of interest (currently, a chat room)
+
+EDU (Ephemeral Data Unit):
+  A message that relates directly to a given pair of home servers that are
+  exchanging it. EDUs are short-lived messages that related only to one single
+  pair of servers; they are not persisted for a long time and are not forwarded
+  on to other servers. Because of this, they have no internal ID nor previous
+  EDUs reference chain.
+
+Event:
+  A record of activity that records a single thing that happened on to a context
+  (currently, a chat room). These are the "chat messages" that Synapse makes
+  available.
+
+PDU (Persistent Data Unit):
+  A message that relates to a single context, irrespective of the server that
+  is communicating it. PDUs either encode a single Event, or a single State
+  change. A PDU is referred to by its PDU ID; the pair of its origin server
+  and local reference from that server.
+
+PDU ID:
+  The pair of PDU Origin and PDU Reference, that together globally uniquely
+  refers to a specific PDU.
+
+PDU Origin:
+  The name of the origin server that generated a given PDU. This may not be the
+  server from which it has been received, due to the way they are copied around
+  from server to server. The origin always records the original server that
+  created it.
+
+PDU Reference:
+  A local ID used to refer to a specific PDU from a given origin server. These
+  references are opaque at the protocol level, but may optionally have some
+  structured meaning within a given origin server or implementation.
+
+Presence:
+  The concept of whether a user is currently online, how available they declare
+  they are, and so on. See also: doc/model/presence
+
+Profile:
+  A set of metadata about a user, such as a display name, provided for the
+  benefit of other users. See also: doc/model/profiles
+
+Room ID:
+  An opaque string (of as-yet undecided format) that identifies a particular
+  room and used in PDUs referring to it.
+
+Room Alias:
+  A human-readable string of the form #name:some.domain that users can use as a
+  pointer to identify a room; a Directory Server will map this to its Room ID
+
+State:
+  A set of metadata maintained about a Context, which is replicated among the
+  servers in addition to the history of Events.
 
 User ID:
-  An opaque ID which identifies an end-user, which consists of some opaque 
-  localpart combined with the domain name of their home server. 
+  A string of the form @localpart:domain.name that identifies a user for
+  wire-protocol purposes. The localpart is meaningless outside of a particular
+  home server. This takes a human-readable form that end-users can use directly
+  if they so wish, avoiding the 3PIDs.
+
+Transaction:
+  A message which relates to the communication between a given pair of servers.
+  A transaction contains possibly-empty lists of PDUs and EDUs.
 
 
 .. Links through the external API docs are below