Fugue

There are several concepts associated with Fugue:

Subject - this is the main theme, the first thing you hear played/sung by the first instrument/voice to enter.

Real Answer/Tonal Answer - this is the subject as played by the second instrument/voice to enter, in a related key. If the intervals between the notes are the same, it's a Real Answer, if one or two intervals have to be changed to make it fit, it's a Tonal Answer

Countersubject - after the subject or answer is played, the continuation of that same instrument or voice is called the countersubject.

Episode - a modulating link between entries of the subject, often based on fragments from the subject or Counter subject.

Exposition - the first part of the fugue, in which each of the voice/instrument parts has stated the Subject or Answer once.

Stretto - when each part enters quickly one after the other. Used to add tension/excitement, usually towards the end of the Fugue.