cell
A cell is the smallest piece of life — the tiniest thing that still counts as alive. It can take in energy, grow, respond to the world around it, and make copies of itself, all on its own.
Some living things are just a single cell, like the bacteria on your skin. Others are built from trillions of them: your own body holds roughly 37 trillion cells, each one a tiny worker doing its part.
Every cell is wrapped in a thin skin called a membrane that decides what gets in and out. Inside sits its genetic instructions and a crowd of tiny machinery that keeps it running. Where those instructions are kept varies — animal and plant cells tuck their DNA inside a walled-off nucleus, while bacteria let theirs float freely with no nucleus at all.
The word comes from the Latin cella, meaning a small room. In 1665 Robert Hooke peered at a sliver of cork through an early microscope, saw rows of tiny empty boxes, and thought they looked like the bare little rooms monks lived in — so he called them cells. The name stuck.