Posture was the way one carried oneself, particularly the orientation of one's spine.
Bad posture, on the other hand, could lead to back problems, such as a pinched nerve. In 2373, after Odo had become humanoid, Doctor Julian Bashir diagnosed the former changeling with a pinched nerve resulting from his bad posture. Odo seemed surprised, as he usually sat straight up in his chair, until Bashir explained that he was too rigid. When Quark saw Odo in pain, he blamed bad posture, too. (DS9: "The Begotten")