I. (ˌ)ənˈles sometimes ə n- conjunction
Etymology: Middle English unlesse (than), unlesse ( that ), alteration (influenced by un- ) (I) of onlesse (than), onlesse ( that ), from on + lesse less — more at on , less
1. : under any other circumstance than that : except on the condition that : if … not
would have been destroyed unless a regiment … had been sent — J.J.Chapman
modern man is obsolete … unless he can stop world wars — Stuart Chase
no person shall be convicted of treason unless on the testimony of two witnesses — U.S. Constitution
2. : without the accompanying circumstance or condition that : but that : but
never a day goes by unless at least one collision occurs — Priscilla Hughes
II. preposition
Etymology: Middle English unlesse ( than ), alteration of onlesse ( than ), from on + lesse
: except possibly : except
no one ( unless the psychotherapists who have to deal with the casualties) seems seriously to have considered the dangers — New Republic