I’m personally not a big fan of this term. However, for ease of standards, I’m using. Just note that an increment is simply a slice of time. That pointed out; here’s the official Scrum Guide wording...
An Increment is a concrete stepping stone toward the Product Goal. Each Increment is additive to all prior Increments and thoroughly verified, ensuring that all Increments work together. To provide value, the Increment must be usable.
Multiple Increments may be created within a Sprint. The sum of the Increments is presented at the Sprint Review, thus supporting empiricism. However, an Increment may be delivered to stakeholders before the end of the Sprint. The Sprint Review should never be considered a gate to releasing value.
Work cannot be considered part of an Increment unless it meets the Definition of Done.