Detection
In general, detection is the extraction of intelligence from a carrier signal in a communication system. Note that this may be either an overt signal, as in a conventional radio broadcast, or a covert signal, as in steganography.
In opto-electronic systems, the generation of an electrical signal in response to a received optical input. For example, the optical signal received from an optical fiber is converted to an electrical signal in a detector, often a photodiode.
In radio systems, the extraction of an AM signal from its carrier frequency.
In steganography, attemps to detect encoded intelligence from suspected carrier material is refered to as steganalysis. Steganalysis has an interesting difference from most other types of detection, in that it can often only produce the probability of the existance of payload material encoded in the carrier; this is in contrast to the detection of signals which are simply encrypted, as the ciphertext can often be detected with certainty, even if it cannot be decoded.
The art of detection a.k.a. following clues, is the work of any detective.