The adjective peripheral is relating to the edge of something, outer part of the field of vision or of secondary importance, not as important as anyone else. It can also be a noun, which means a piece of equipment connected to a computer such as printer.
Synonyms are outlying, border, incidental, or immaterial.
The word origins from periphery + -al. An earlier formation was peripherial (1670s). Its first known use as an adjective was during the 1800s and as a noun in 1966.
I saw him smiling on my peripheral vision.
Education shouldn’t be peripheral.
Please move that peripheral equipment to my table.