Access point AP
The device that turns wired Ethernet into Wi-Fi. In a home network it lives inside the router; in any larger installation it's a separate box in the ceiling, powered over PoE.
Several access points share the same SSID but each has its own BSSID — the client itself decides when to switch, which is why roaming sometimes works badly despite perfect coverage. The 802.11k, v and r standards exist precisely to help the client pick well and switch fast enough for an ongoing call. The key to good coverage is rarely more transmit power but channel planning: 2.4 GHz has only three non-overlapping channels, and the neighbour's network interferes more than the walls do. A central controller — physical, software or cloud-based — handles configuration and channel selection in larger deployments. Mesh is the fallback when cable can't be pulled; each hop roughly halves capacity.