The reduction in your ownership percentage when new shares are issued.
Own 100% and issue 20% to an investor, and you now own 80%. Nobody took anything from you; the pie got bigger and your slice is a smaller fraction of it.
Dilution is not inherently bad — 60% of a funded company beats 100% of one that ran out of money. It is bad when it is unmodelled. Two SAFEs, an option pool carved out pre-money, and a priced round can compound to a founder stake well below what anyone in the room intended, and each individual step looked reasonable.
The habit worth forming: before signing anything that issues or promises shares, model the fully-diluted cap table afterwards.
Project types where this term stops being vocabulary and starts being a decision you have to make.