1. an abrupt, involuntary interruption in the flow of thought or speech in which the individual is suddenly aware of not being able to perform a particular mental act, such as finding the words to express something he or she wishes to say. Also called mental block. See retrieval block; tip-of-the-tongue phenomenon.

2. in psychotherapy, an obstacle to progress that is perceived as a barrier that cannot be crossed.

3. any physical, biochemical, or psychological barrier or obstacle that obstructs or impedes a process, function, or activity.

4. a group or subset of study participants who share a certain characteristic and are treated as a unit in an experimental design.

5. a set of variables entered as a single entity into a regression analysis or similar statistical procedure.