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.

Instantaneously developing muscular stiffening that occurs at the moment of death in individuals who are engaged in strenuous physical activity immediately prior to their demise.

The detection of this phenomenon is important because the body maintains the postion it was in at the time of death and this positioning continues until true rigor mortis develops.