Metrics
Reach
Reach is the number or percentage of distinct people exposed to a campaign at least once during a period. It answers 'how many people did we get to', distinct from impressions, which count deliveries, and frequency, which counts repetitions per person.
In practice
Reach is the primary currency of awareness objectives, usually paired with a frequency target - reaching 60% of the audience at an average of three exposures says far more than either number alone.
Cross-platform reach is harder than it looks: each platform deduplicates within its own walls, so summing platform reach figures overstates the true unduplicated number.