at sight
1. At or upon the first instance something is seen or viewed. Don't trespass in this part of the country, you might be shot at sight! I've heard so much about her that I think I'd know her at sight.
2. Upon presentation; immediately; at once. Especially used in the context of commerce. The idea of having credit at a shop has largely gone by the wayside; nowadays almost everything you buy is payable at sight.
at sight
1. See on sight.
2. On presentation, especially a draft for payment. For example, These bills are payable at sight. This usage replaced on sight in the late 1600s.