Doc: define 'Ballpark transformation'

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diff --git a/docs/source/glossary.rst b/docs/source/glossary.rst
index 21dd0f34..883460d4 100644
--- a/docs/source/glossary.rst
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@@ -6,6 +6,25 @@ Glossary
 
 .. glossary::
 
+    Ballpark transformation
+
+        For a transformation between two geographic CRS, a ballpark
+        transformation is a coordinate operation that only takes into account
+        potential difference of axis orders (long-lat vs lat-long),
+        units (degree vs grads) and  prime meridian (Greewich vs Paris/Rome/other
+        historic prime meridians). It does not attempt any datum shift, hence
+        the "ballpark" qualifier in its name. Its accuracy is unknown, and could
+        lead in some cases to errors of a few hundreds of metres.
+
+        For a transformation between two vertical CRS or a vertical CRS and
+        a geogrpahic CRS, a ballpark transformation only takes into account
+        potential different in units (e.g. metres vs feet). Its accuracy is
+        unknown, and could lead in some cases to errors of a few tens of metres.
+
+        .. note::
+
+            The term "Ballpark transformation" is specific to PROJ.
+
     Pseudocylindrical Projection
 
         Pseudocylindrical projections have the mathematical characteristics of