Cell
A cell is the smallest unit of life, a membrane-bound package of cytoplasm that carries out every process an organism needs to survive.
Definition
A cell is the smallest living building block of every plant, animal, and person: a thin membrane wrapped around a jelly-like fluid in which the work of staying alive happens. It is the basic structural and functional unit of all organisms, and every cell has a cell membrane, cytoplasm, and genetic material (DNA) with which it obtains energy, grows, responds to its surroundings, and reproduces. Seen more closely, a cell is a self-maintaining, membrane-bounded compartment in which metabolism, information storage, and reproduction are coupled. Its phospholipid bilayer separates an internal chemistry that is far from equilibrium from the outside world, and it spends energy carriers such as ATP to drive synthesis, transport, and movement. Cells are classified as prokaryotic, with no membrane-bound nucleus, or eukaryotic, with a nucleus and membrane-bound organelles.
Example
Your body is made of trillions of cells, and a single skin cell is too small to see without a microscope, yet it can grow, use energy, and make copies of itself. A typical animal cell is about $10$ to $30\ \mu\text{m}$ across, a bacterial cell only $1$ to $5\ \mu\text{m}$, and a red blood cell about $7\ \mu\text{m}$, so roughly $140$ red cells side by side would span $1\ \text{mm}$. Size matters because a cell feeds itself through its surface: a cube-shaped cell with sides of $1\ \mu\text{m}$ has a surface-area-to-volume ratio of $6\ \mu\text{m}^{-1}$, while a cube with sides of $10\ \mu\text{m}$ has a ratio of only $0.6\ \mu\text{m}^{-1}$. That tenfold drop is one reason large eukaryotic cells rely on internal membranes and active transport rather than diffusion alone.
Key Insight
Everything alive is made of cells, and nothing smaller than a cell is alive on its own; a cell is like a tiny factory that runs itself. Cells stay small because they must exchange materials through their surface, and as a cell grows its volume increases faster than its surface area, so a very large cell could not feed itself or clear waste quickly enough. At the deepest level a cell is a chemical system that keeps itself ordered by constantly spending energy, which is consistent with the second law of thermodynamics because the cell exports entropy as heat and waste. Life at the cellular level is a steady state, not a static structure.