A thoughtprint is unconscious communication hidden within verbal or written communication. A more complete definition can be found here.
Various types of communications can be profiled to reveal any subconcious messages that are contained in them. Communication can be oral or written. Typical communications are: ransom notes, letters, suicide notes, police interrogations, etc.
Thoughtprint decoding can also be useful in a defense case. The subconscious can also give clues to innocence.
More information on how thoughtprints are used is available here.