MLC, or Multi-level Cell, refers to the type of NAND used on an SSD. SSDs were initially made with Single-level Cell (SLC) NAND. Although SLC was extremely fast and durable, it was very expensive. As technology matured, NAND manufactures were able to store more bits of data in the same amount of space, known as Multi-Level Cell. MLC sacrifices a small amount of performance and endurance, while increasing capacity and lowering cost-per-Gigabyte.