耳麦 is a blend of
耳机 (ěrjī, headphones) +
麦克风 (màikèfēng, microphone), so it specifically means a headset with a built-in mic. Compare with
耳机 (just headphones, no mic),
蓝牙耳机 (lányá ěrjī, Bluetooth earbuds), and
入耳式耳机 (rù'ěrshì ěrjī, in-ear earphones).