What is a USB Flash Drive?
A USB flash drive, also called a thumb drive, USB stick, pen drive, USB flash disk, memory stick, disk on key, jump drive, mobile disk, or clip drive, is a small portable device where files are saved (it's mass storage device) that is plugged into a USB port of a computer.
In the past, we used floppy disks, then CDs to transfer files from one computer to another. Nowadays, we use USB sticks.
The floppy goes into the floppy disk drive, the CD goes into the CD drive, the hard disk is contained within a hard disk drive, and the SSD is a solid state drive that stores data in non-volatile flash memory. The USB flash drive means that it also uses this technology called "flash" memory to physically store the data. This means an SSD is just a big, non-portable pen drive.
Observations
The term "USB flash drive" used to be more common in the past. According to Google Books NGram Viewer, "thumb drive" is the prevailing term as of 2022, with USB stick after it, USB flash drive in third place and falling, and pen drive in last place and rising.