LCL is an abbreviation of 'lateral collateral ligament'.
LCL - a structure of the posterolateral corner
The side of the knee towards the back is anatomically complex, and is referred to as the posterolateral corner, of which the LCL is a part.
Lateral Collateral Ligament Injury About the Knee: Anatomy, Evaluation, and Management. Grawe B, Schroeder AJ, Kakazu R and Messer MS. J Am Acad Orthop Surg. 2018 Mar 15;26(6):e120-e127. doi: 10.5435/JAAOS-D-16-00028.
Anatomic Fibular Collateral Ligament Reconstruction. Moatshe G, Dean CS, Chahla J, Cruz RS and LaPrade RF. Arthrosc Tech. 2016 Apr; 5(2): e309–e314.