Fugue Glossary Fugue Diagram   Theory Things   William Wieland
Fugue an imitative contrapuntal composition that develops a theme (Fugues usually incorporate three or four voices which enter one by one.)
Subject (S) the recurring theme of a fugue (The subject is stated at the very beginning of a fugue, usually unaccompanied. The subject is often in two parts, the head and the tail.)
Answer (A) the second (and sometimes fourth) statement of the subject transposed to the dominant (up a perfect 5th or down a perfect 4th)
Real Answer
Tonal Answer
an exact transposition of the subject (up a P5 or down a P4)
a transposed subject which is altered to maintain the tonic key 
Countersubject
(CS)
the theme which regularly accompanies the subject (Not all fugues have countersubjects. Countersubjects usually follow the initial subject statement in the same voice and therefore first accompany the answer.)
Exposition a section in which the complete subject appears, particularly the initial section which introduces the subject successively in each voice
Bridge a short passage in the exposition which links entries of the subject (a.k.a. link or codetta)
Episode a section which does not include the complete subject (Episodes often employ sequences or stretto.)
Sequence a technique in which a motive or short figure is successively stated, the motive being transposed by equidistant intervals
Stretto the introduction of two or more entries of the subject at a closer time-interval than that of the exposition to heighten the dramatic effect
False Entry an incomplete subject entry often used in stretto
The San Francisco Conservatory Department of Musicianship and Music Theory has an excellent slide show entitled Fugue: Systematized Polyphony.