Except on a specified condition; if not. / If not; used with counterfactual conditionals. / Except if; used with hypothetical conditionals.

Obsolete spelling of unless

(mathematics, logic) implies a negative and is implied by a negative; i.e. B implies not-A and not-B implies A

(mathematics, logic) Precisely unless.

uneducated; ignorant

(US, dialect) unless

Not lessened.

Not making or becoming less.