diff --git a/certbot/cli.py b/certbot/cli.py
index fea83da29..06b95b43b 100644
--- a/certbot/cli.py
+++ b/certbot/cli.py
@@ -904,7 +904,7 @@ def prepare_and_parse_args(plugins, args, detect_defaults=False):  # pylint: dis
              "'run' subcommand this means reinstall the existing cert). (default: Ask)")
     helpful.add(
         "automation", "--expand", action="store_true",
-        help="If an existing cert covers some subset of the requested names, "
+        help="If an existing cert is a strict subset of the requested names, "
              "always expand and replace it with the additional names. (default: Ask)")
     helpful.add(
         "automation", "--version", action="version",
diff --git a/docs/cli-help.txt b/docs/cli-help.txt
index 91041458e..3154bbda6 100644
--- a/docs/cli-help.txt
+++ b/docs/cli-help.txt
@@ -99,7 +99,7 @@ automation:
                         keep the existing one until it is due for renewal (for
                         the 'run' subcommand this means reinstall the existing
                         cert). (default: Ask)
-  --expand              If an existing cert covers some subset of the
+  --expand              If an existing cert is a strict subset of the
                         requested names, always expand and replace it with the
                         additional names. (default: Ask)
   --version             show program's version number and exit