Other forms: overhears, overheard, overhearing
To overhear is to hear something you were not meant to. Overhear a conversation, I overheard them, overheard by accident.
The accident is essential: overhearing is unintentional, and eavesdropping is deliberate — which is the whole difference and a distinction people rely on when explaining themselves. It takes a direct object or a clause. Note that the Old English word meant both to overhear and to disobey, since not listening properly and listening too well were expressed by the same over-.
1verboverhearTo hear something you were not meant to.
“I couldn't help overhearing your conversation on the train.”
overhear is an unintended hearing.
Old English oferhieran meant to overhear and also to disregard. The German überhören still means to fail to hear, so the same compound went opposite ways in the two languages.
Old English
oferhīeran — to overhear, hear, disobey, disregard, neglect
Middle English
overheren
English, today
overhear
From Middle English overheren, from Old English oferhīeran (“to overhear, hear, disobey, disregard, neglect”), equivalent to over- + hear. Cognate with Dutch overhoren (“to hear, hear about”), German überhören (“to not hear, ignore”), Danish overhøre (“to overhear”), Icelandic yfirheyra (“to hear”), Gothic 𐌿𐍆𐌰𐍂𐌷𐌰𐌿𐍃𐌾𐌰𐌽 (“to disregard, disobey”) (in 𐌿𐍆𐌰𐍂𐌷𐌰𐌿𐍃𐌴𐌹𐌽𐍃).
Etymology from Wiktionary, CC BY-SA