1. Descriptive of something that happens without delay; prompt: After Jack took his medicine, relief was instantaneous.
2. Referring to that which is done or made as quickly or directly as possible: Jake made an instantaneous or immediate reply to Jim's e-mail.
3. Pertaining to a condition which is present or occurs at a specific time: Sam had an instantaneous pain in his thumb when he hit it with the hammer while he was pounding nails into the wooden box.