doc: add a glossary

Change-Id: I3bf4d6be8996c3ede0ae58dd14c054bd53982659

doc/source/glossary.rst

@@ -0,0 +1,33 @@
+========
+Glossary
+========
+
+.. glossary::
+
+   Resource
+     An entity representing anything in your infrastructure that you will
+     associate metric(s) with. It is identified by a unique ID and can contain
+     attributes.
+
+   Metric
+     An entity storing measures identified by an UUID. It can be attached to a
+     resource using a name. How a metric stores its measure is defined by the
+     archive policy it is associated to.
+
+   Measure
+     A datapoint tuple composed of timestamp and a value.
+
+   Archive policy
+     A measure storage policy attached to a metric. It determines how long
+     measures will be kept in a metric and how they will be aggregated.
+
+   Granularity
+     The time between two measures in an aggregated timeseries of a metric.
+
+   Timeseries
+     A list of measures.
+
+   Aggregation method
+     Function used to aggregate multiple measures in one. For example, the
+     `min` aggregation method will aggregate the values of different measures
+     to the minimum value of all the measures in time range.

doc/source/index.rst

@@ -67,5 +67,6 @@ Documentation
    rest
    statsd
    grafana
+   glossary
 
 .. _`OpenStack`: http://openstack.org