unable to hear or speak: deaf, mute, or both
This word describes a person cut off from ordinary communication, whether the block is in the ears or in the tongue. Its root idea is something "blunt" or "dulled," a faculty that will not work. In everyday Greek it named the deaf, the mute, or someone who was both, and the New Testament uses it in all of these ways.
The reason one word covers two conditions is that the ancient world saw them as a single affliction: a person deaf from birth never learns to speak, so deafness and muteness were understood to travel together. That is why the same word can mean "the deaf hear" in one verse and "the mute spoke" in another. Where a writer needed to be exact, Greek reached for narrower words, ἄλαλος for one who simply cannot speak and μογιλάλος for one whose speech is labored.
In the Gospels the word almost always turns up in a healing scene, and it carries the weight of Isaiah's promise that in God's coming reign "the ears of the deaf shall be unstopped" and "the tongue of the mute shall sing." Isaiah's Hebrew uses the deaf word חֵרֵשׁ and the mute word אִלֵּם; when Jesus opens ears and looses tongues, the Gospel writers are pointing back to that promise.