terra firma

NOUN /,tɛrə_'fɜrmə/
11 LETTERS · 3 SYLLABLES · STARTS WITH T

terra firma is a valid 11-letter word worth 15 points in Scrabble and 16 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
T1E1R1R1A1F4I1R1M3A1
1 + 1 + 1 + 1 + 1 + 0 + 4 + 1 + 1 + 3 + 1 =15 PTS
DOUBLE WORD → 30TRIPLE WORD → 45
MEANING

What does terra firma mean?