A cell is a a small, functional organism.
Cell's are microscopic, meaning that there not visable to the human eye.(Unless you are using a microscope.)
Most microscopic organisms have one cell. Which are either eukaryotic or prokaryotic.
Microscopic organism's normaly consistsof a cytoplasm and a nucleus enclosed in a membrane.
Also a cell, is the smallest unit of life
I hope this help's you understand the definition of a cell.