MLC
Multi-Level Cell — NAND flash that stores 2 bits per cell via 4 voltage levels. Twice as dense as SLC, roughly half the endurance and speed. Today mostly historical — consumer SSDs have moved on to TLC and QLC.
Endurance: SLC ~100k erase cycles, MLC ~10k, TLC ~3k, QLC ~1k. Write speed drops dramatically with TLC/QLC because more precision is required. SLC-cache trick: use the first X GB as SLC (fast write), defrag to TLC/QLC in the background. Confusion: "MLC" is sometimes used as an umbrella term for all multi-bit (including TLC, QLC). Strict definition is 2 bits. Enterprise SSDs (Samsung PM/Intel D series) use eMLC or endurance-graded TLC for long life.