It is several strikes within a single ability activation, against one or more targets.

Examples: Tiros's basic attack “Mace Strike,” Siri's 2nd special attack “Blade Weave,” Astoria's 2nd special attack “Star Rays.”

Artifacts: if a multi-attack targets a single target, all hits are amplified.

If a multi-attack targets a primary target + additional targets, only the primary target receives increased damage.