The diameter is from 1 side to the other far side. (Straight across).
60 Inches
Round tables are always measured & advertised in diameter.
Standard seating at a 60 inch round table is 8 people, which allows for 23.5" in width per chair. (around the perimeter).
Some people say you can fit 10 chairs at a 60 inch round table, which allows 18.8" in width for the chair. A chair might fit in 18.8" of space. The average person is wider than 18.8", so more than 8 people at a 60 inch is NOT the best practice.