You mean an 18650? An 18650 is a small cell, in the neighborhood of the size of an AA battery, that is used in all kinds of electronics, including small consumer electronics. They can be used as a basic building unit for larger battery packs – if you take apart a lithium powerstation, you can probably disassemble it and find 18650s. It may be that some EVs build packs from 18650s, and I’m sure that ebikes will, but they aren’t specific to EVs.