Expansion revenue is the additional ARR generated from your existing customer base during a period - through buying more seats, upgrading to higher tiers, adding products, or accepting price increases. It's a critical input to NRR; without it NRR can't exceed 100%. The mix between upsell-driven expansion and price-increase expansion is a quality signal.
See how public B2B SaaS companies disclose this metric, with full historical time series.