1. in testing, conditioning, or other experimentation, a single performance of a given task (e.g., one run through a maze) or a single presentation of a stimulus (e.g., one viewing of an ordered list of three-letter words).

2. see clinical trial; treatment trial.

3. in parapsychology research, any single attempt by a participant to identify a target by clairvoyance or telepathy or to influence a target by psychokinesis. In experiments using Zener cards, each turn of a card is therefore a separate trial.