Enzyme
Definition
A protein that speeds up biological reactions: the catalysts of life, each specialized for one specific job.
Technical definition
An enzyme is a biological catalyst, usually a protein (some are RNA), that accelerates reactions by lowering activation energy through an active site complementary to the substrate. Enzymes are highly specific, work under mild conditions, and are regulated by inhibitors, pH, and temperature.
Example
Amylase in your saliva starts digesting starch the moment you chew. Without it, the reaction would take far too long to be useful.
Common confusion
Enzymes are not used up, but they can be damaged. Boiling an egg denatures its proteins permanently, which is why cooked egg white never turns clear again.