M.2
Small form factor for SSDs (and Wi-Fi cards) — directly on the motherboard, no cable.
Replaced mSATA. Sizes: 2280 (most common, 22×80mm), 2230 (short, Steam Deck). Two protocols: SATA (slow, old) and NVMe (fast, modern). Check that the motherboard supports the right one — not all M.2 slots are equal.