noun

1. an expanding or being expanded; enlargement; dilation
2. an expanded thing or part
3. the amount, degree, or extent of expansion
4. a development or full treatment, as of a topic
5. the process or result of working out or giving the full form of a contraction, equation, etc.
6. Mech. the expanding in volume of steam in the cylinder of a steam engine after cutoff, or of gas in the cylinder of an internal-combustion engine after explosion

Etymology: LL expansio < L expansus: see expanse