A wireless adapter is a hardware device that is generally attached to a computer or other workstation device to allow it to connect to a wireless system. Before the advent of consumer devices with built-in Wi-Fi connectivity, devices required the use of wireless adapters to connect to a network.
Wireless adapters are also known as Wi-Fi adapters.
Techopedia explains Wireless Adapter
Wireless adapters often come in USB stick form, which must be plugged into a USB port of a computer or device. USB has become a universal standard for all sorts of accessory devices that plug into a computer or workstation device. Wireless adapters may also come in the form of PCI network cards that plug into a PCI slot on the computer motherboard. They do not generally plug into the Ethernet port. Instead, an Ethernet cable can connect a computer directly to a router or other device.
New technology has made wireless adapters somewhat obsolete, as new generations of portable computers generally have built-in wireless connectivity that can be enabled or disabled as needed.