I'd suggest "enterprise" means "everywhere the classic top-down enterprise sales strategy works", which generally means "companies over 1000 employees". At Expensify, we define the market as:

1 - Individual, prosumer, contractor, solopreneur, etc. 2-10 - "Very small business" 11-100 - "Small business" 101-1000 - "Midsize business" 1001+ - "Enterprise"

But you're very right to ask -- different industries segment the market differently. We focus on employee headcount because we sell seats; companies who sell big contracts not tied to seats typically segment the market by revenue.