A connector that can use many different buses, ranging from SATA to mini PCIe. Common uses include SSDs and Wireless/Bluetooth expansion cards.
M.2, formerly known as the Next Generation Form Factor (NGFF), is a specification for internally mounted computer expansion cards and associated connectors.
There are four commonly used keys, A, B, E, and M.
B and M are commonly used for SSDs, A and E are used for WiFi/Bluetooth as well as cellular cards.