背后 literally means 'back + behind' and functions both as a noun meaning 'the area behind something or someone' (e.g., the physical space at the rear) and as an adverb meaning 'behind one’s back' in figurative, often negative contexts—such as gossiping or acting secretly without someone’s knowledge. As a noun, it’s location-based and commonly used with measure words like '面 (miàn)' or '处 (chù)', e.g., '背后有一棵树' (There’s a tree behind [it]).

As an adverb, 背后 emphasizes hidden or covert action: when people do something behind someone’s back, they intend to conceal it from that person, usually implying deception, disloyalty, or lack of transparency. It frequently appears in journalistic, conversational, or moral discourse. Note that while the characters individually mean 'back' and 'behind', together they carry strong contextual nuance—never neutral in the figurative sense.