"A verbal operant in which a response of given form is evoked (or at least strengthened) by a particular object or event or property of an object or event" -- B.F. Skinner, 1957
A tact is a voicing of a word in response to a non-verbal object or attribute in the environment. Or to put it more simply, a tact is the naming of an object, action, event, etc. This is one of the stages of language learning in children. And is often simply referred to as labeling.
This stage follows the echoic stage, in which the child merely repeats what e hears. Tacting is different from manding; a mand is a response to the child's desire for an object, while a tact is a response to the actual object. Tact is also different from interverbal, in that an interverbal is specifically in response to spoken language, while tact is in response to non-verbal objects and conditions.
If you wish to reinforce the use of tacts by a child, do not reward the use of a word by presenting the child with the referred-to object; that would reinforce the use of the word as a mand. Instead, use verbal or kinesic rewards -- "That's right, that's a cat!", or a smile.
Tacts are of interest to behavioral psychologists and speech therapists. Other things in the same vein and of interest to the same people are Mand, Echoic, Intraverbal, Receptive repertoire, and Imitation.