is,
Strength of Mind.
Fortitude is that strength of mind or soul which enables a person to be patient and long-suffering, kind and courageous, faithful and just.

Be strong and of good courage, fear not, nor be afraid of them: for the Lord thy God, He it is that doth go with thee; He will not fail thee nor forsake thee.
Deuteronomy 31:6

For God hath not given us the spirit of fear; but of power, and of love, and of a sound mind.
2 Timothy 1:7