A 25 Torx bit is 25 What???

Torx bits come in sizes like 15, 25, etc. You may use a 60 for brake calipers. My question is What does the number stand for? A 25 Torx bit can't be 25 mm, that would be an inch. Is it 25/100 inch? I have asked this question to many people, but never got an answer.