In
experiments with live animals, this refers to an experimental setup from which the animal does not recover.
In an "acute prep", the animal (rat, monkey, rabbit, etc.) is first put under anesthesia and then subjected to a surgical procedure or experimental protocol, such as brain or muscle electrophysiology, application of an experimental medical device, or injection with a novel or experimental drug agent. The animal is typically kept under anesthetic at all times.
At the conclusion of the experiment, the animal is sacrificed in one of several ways: anesthetic overdose, decapitation, transcardial perfusion (i.e. introducing a fixative into the heart in order to preserve all of the body tissues), or cervical dislocation. Sometimes the entire body or selected tissues are then preserved with formalin or with a flash freezing process (depending on the sort of analysis to be performed on them.)
Contrast this with a "chronic prep": a procedure in which the animal recovers after the surgery. This may be done in order to observe changes in behavior due to the procedure or to allow the animal to be used in another preparation later on.