Permanent is a related term of undecaying.
As adjectives the difference between permanent and undecaying
is that
permanent is without end, eternal while
undecaying is that does not decay; lasting, imperishable.
As a noun permanent
is a chemical hair treatment imparting or removing curliness, whose effects typically last for a period of weeks; a perm.
As a verb permanent
is (dated) to perm (the hair).