Availability and uptime are used interchangeably in most conversations, and for a single service they are the same calculation. The distinction only starts to matter once you are composing services together.
Worth knowing
Composite availability is the part people get wrong. Chain two services that are each 99.9% available and, if either failing takes you down, the combined figure is roughly 99.8%. Dependencies multiply. A system built from five three-nines components is a two-and-a-half-nines system unless you have built redundancy between them.