A temporal particle that marks the boundary or limit of an action in time. In its dominant sense, ἕως means 'until' — pointing to the endpoint at which something ceases or a new event begins, as in 'until the day' or Matthew 1:25's 'until she bore a son.' Less commonly it shades into 'while,' expressing ongoing duration rather than termination, as in Jesus' instruction to pray 'while I go over there' (Mark 14:32). Spanish distinguishes these neatly: 'hasta' (until) versus 'mientras' (while), confirming the semantic split visible in the Greek usage.