The 2020 Scrum Guide defines an Increment as “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… The entire Scrum Team is accountable for creating a valuable, useful Increment every Sprint.”
A Product Increment is the sum of all Product Backlog items completed during a Sprint, combined with all previous Sprint increments. The increment must be usable, meaning it should work as intended. This aligns with the Scrum principle of delivering "Done" increments potentially releasable. Usability is a key aspect of the Definition of Done in Scrum.