means A solemn, binding promise — usually one made with great seriousness, often sacred or ceremonial — committing you to do or refrain from something.

from From Old French 'vou' (a vow, promise), drawn from Latin 'votum,' meaning a vow or wish — literally 'a thing solemnly promised,' from 'vovere,' to pledge or dedicate. That same Latin root quietly powers 'vote' (a wish expressed) and 'devote' (to vow oneself wholly to). So a vow began as something offered up — a promise pressed toward the heavens, hoping they were listening.