docs: add glossary.md

Documentation/0.5/glossary.md

@@ -0,0 +1,31 @@
+## Glossary
+
+This document defines the various terms used in etcd documentation, command line and source code.
+
+### Node
+
+Node is an instance of raft state machine.
+
+It has a unique identification, and records other nodes' progress internally when it is the leader.
+
+### Member
+
+Member is an instance of etcd. It hosts a node, and provides service to clients.
+
+### Cluster
+
+Cluster consists of several members.
+
+The node in each member follows raft consensus protocol to replicate logs. Cluster receives proposals from members, commits them and apply to local store.
+
+### Peer
+
+Peer is another member of the same cluster.
+
+### Client
+
+Client is a caller of the cluster's HTTP API.
+
+### Machine (deprecated)
+
+The alternative of Member in etcd before 0.5