Fortitude

Affects the duration of received negative effects.

According to developer's explanation in Discord Game Designer's comments on the official Stoneshard Discord server (in Russian), a Fortitude value of 100% reduces the duration of negative effects by half. If this results in a fractional value, it gets rounded to the nearest integer, i.e. 1.5 is rounded to 2. The duration of any negative effect can't be reduced below 1.