ACL is an abbreviation for 'anterior cruciate ligament'.
View of ACL from the front of the bended knee.
View of the ACL from the side, with the knee cut in half.
The anterior cruciate ligament
The anterior cruciate ligament is one of the two large ligaments right inside the knee joint, which are both important for knee stability.
A Systematic Summary of Systematic Reviews on the Topic of the Anterior Cruciate Ligament. Anderson MJ, Browning WM, III, Urband CE, Kluczynski MA and Bisson LJ. Orthop J Sports Med. 2016 Mar; 4(3): 2325967116634074.
What I have learned about the ACL: utilizing a progressive rehabilitation scheme to achieve total knee symmetry after anterior cruciate ligament reconstruction. Shelbourne KD and Klotz C. J Orthop Sci. 2006 May; 11(3): 318–325.