In general, detection is the action of accessing information without specific cooperation from with the sender.
The term is still in use today to describe a component that extracts a particular signal from all of the electromagnetic waves present.
In optoelectronics, "detection" means converting a received optical input to an electrical output.
In steganography, attempts to detect hidden signals in suspected carrier material is referred to as steganalysis.
In the military, detection refers to the special discipline of reconnaissance with the aim to recognize the presence of an object in a location or ambiance.