non-regular
/nɒnˈrɛɡjʊlə/adjective
1That is not usual or habitual; that does not occur regularly; that does not follow the usual rule or pattern; nonstandard.
2 Military = irregular.
noun
1 Military = irregular.
2A person who is not a regular customer, contributor, participant, visitor, etc.; (specifically in Sport ) a person who does not play regularly in a particular team.
Origin
Late 19th century; earliest use found in Annals of Mathematics. From non- + regular.