U.2 SSD
An SSD form factor (SFF-8639) in a 2.5-inch enclosure carrying NVMe over PCIe — so NVMe performance but in a server-drive form with hot-swap. Common in data centers where M.2 is too small, hot, and hard to replace in operation.
Win over M.2 in a server: better cooling (a 2.5-inch enclosure with surface area), high capacity (many TB), hot-swappable via a backplane, often a dual PCIe port for redundancy. Distinct from SATA 2.5-inch (looks alike but U.2 is NVMe/PCIe). The connector can also carry SAS/SATA in the same slot (tri-mode backplanes). On desktop it's unusual but occurs via an M.2-to-U.2 adapter for those who want enterprise SSDs. The server-class successor: EDSFF (E1.S, E3.S "rulers") is gradually taking over to maximize density + cooling. OCuLink is often used to connect U.2 drives.