diff --git a/glossary.tex b/glossary.tex
index bebf408142960ef4cdab588e186e3c2e1dea8aed..d950edb28f191e5ef7837600a91ead0108b297bf 100644
--- a/glossary.tex
+++ b/glossary.tex
@@ -44,6 +44,7 @@
 
 \newglossaryentry{interval}{
   nonumberlist,
+  prefix={an\space},
   name={Interval},
   description={A quarter of a day -- morning, afternoon, evening, or night}
 }