Cliff

Definitions

The initial period (typically one year) of a vesting schedule during which no equity vests. If you leave before the cliff, you get nothing. The startup's insurance against hiring mistakes.
In plain English: A waiting period (usually one year) before you get any of your stock options — if you leave before it's up, you walk away with nothing.
The standard Silicon Valley vesting schedule is 4 years with a 1-year cliff: 25% vests after year one, then the remainder vests monthly or quarterly over the next 3 years. The cliff protects the company from giving equity to short-term hires.
Example: 'I left at 11 months and lost all my equity because of the cliff. Three more weeks and I'd have had 25%. Lesson learned.'
Source: standard terms