diff --git a/Makefile.inc1 b/Makefile.inc1
--- a/Makefile.inc1
+++ b/Makefile.inc1
@@ -2,7 +2,7 @@
 #
 # Make command line options:
 #	-DNO_CLEANDIR run ${MAKE} clean, instead of ${MAKE} cleandir
-#	-DNO_CLEAN do not clean at all
+#	-DWITHOUT_CLEAN do not clean at all
 #	-DDB_FROM_SRC use the user/group databases in src/etc instead of
 #	    the system database when installing.
 #	-DNO_SHARE do not go into share subdir
diff --git a/share/man/man7/build.7 b/share/man/man7/build.7
--- a/share/man/man7/build.7
+++ b/share/man/man7/build.7
@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd August 8, 2025
+.Dd August 20, 2025
 .Dt BUILD 7
 .Os
 .Sh NAME
@@ -801,6 +801,9 @@
 instead of
 .Dq make cleandir .
 .It Va NO_CLEAN
+The legacy name for
+.Va WITHOUT_CLEAN .
+.It Va WITHOUT_CLEAN
 If set, no object tree files are cleaned at all.
 This is the default when
 .Va WITH_META_MODE
@@ -811,12 +814,34 @@
 .Xr src.conf 5
 for more details.
 Setting
-.Va NO_CLEAN
+.Va WITHOUT_CLEAN
 implies
 .Va NO_KERNELCLEAN ,
 so when
-.Va NO_CLEAN
+.Va WITHOUT_CLEAN
 is set no kernel objects are cleaned either.
+.Pp
+Note that some circumstances will cause the object tree to be cleaned despite
+having set the
+.Va WITHOUT_CLEAN
+option.
+If a new
+.Dq build epoch
+is recorded in
+.Pa .clean_build_epoch
+in the root of the source tree that does not match the epoch recorded in the
+file of the same name in the object tree, then a clean will be performed.
+Build epochs are expected to be used sparingly to handle changes that touch
+large parts of the tree in ways that
+.Xr make 1
+cannot handle adequately without the assistance of
+.Va META_MODE
+and
+.Xr filemon 4 .
+.Pp
+The
+.Pa .clean_build_epoch
+will not cause the kernel object tree to be cleaned.
 .It Va NO_CTF
 If set, the build process does not run the DTrace CTF conversion tools
 on built objects.
@@ -859,7 +884,7 @@
 If set, the build target
 .Cm buildworld
 defaults to setting
-.Va NO_CLEAN ,
+.Va WITHOUT_CLEAN ,
 .Va NO_OBJWALK ,
 and will skip most bootstrap phases.
 It will only bootstrap libraries and build all of userland.