The definition I know is "the first ___ dollars you pay out of pocket, until your insurance kicks in". But the insurance always kicks in from the first dollar.
For example, I have a PPO with a $500 annual deductible. I go for my first doctor's office appointment of the year. The bill / EOB says $250, and the "discounted network rate" billed was $125. I'm never billed for $125. Rather, the insurance paid for most of it.