Cartilage was a flexible connective tissue found in many areas in the bodies of Humans and other animals. While not as hard and rigid as bone, it was stiffer and less flexible than muscle.
Yarim Fel Syndrome affected the cartilaginous tissue of Cardassians. (DS9: "Ties of Blood and Water")
Cartilage was also a component of Ferengi ears. In 2369, while giving Quark oo-mox, Vash admired his cartilage. (DS9: "Q-Less")
Bolians' tongues had a cartilaginous lining that would afford protection against even the most corrosive acid. (VOY: "Flashback")
In 2373, while examining the remains of Hogan, Gegen noticed that his cartilaginous microstructure was porous, indicating a warm-blooded non-Saurian. (VOY: "Distant Origin")