PEP 418: Stephen J. Turnbull's patch 3: complete the Glossary

pep-0418.txt

@@ -595,6 +595,10 @@ Glossary
    The amount of deviation of measurements by a given instrument from
    true values. See also `Accuracy and precision
    <http://en.wikipedia.org/wiki/Accuracy_and_precision>`_.
+   Inaccuracy in clocks may be caused by lack of precision, drift, or an
+   incorrect initial setting of the clock (e.g., timing of threads is
+   inherently inaccurate because perfect synchronization in resetting
+   counters is quite difficult).
 
 :Adjusted:
    Resetting a clock to the correct time.  This may be done either
@@ -636,7 +640,9 @@ Glossary
 
 :Drift:
    The accumulated error against "true" time, as defined externally to
-   the system.
+   the system.  Drift may be due to imprecision, or to a difference
+   between the average rate at which clock time advances and that of
+   real time.
 
 :Epoch:
    The reference point of a clock.  For clocks providing <civil time>,
@@ -662,7 +668,9 @@ Glossary
 
 :Precision:
    The amount of deviation among measurements of the same physical
-   value by a single instrument.
+   value by a single instrument.  Imprecision in clocks may be caused by
+   a fluctuation of the rate at which clock time advances relative to
+   real time, including clock adjustment by slewing.
 
 :Process Time:
    Time elapsed since the process began.  It is typically measured in