unless

/ən-ˈles/

Middle English unlesse, alteration of onlesse, from on + lesse less

conjunction

  1. except on the condition that : under any other circumstance than

  2. without the accompanying circumstance or condition that : but that : but

preposition

  1. except possibly : except

idiom

  1. only if

"Will I have to pay?" "Not unless you want to."