The word 'cruciate' means 'crossed'. The 'cruciate' ligaments are so-called because they cross over one another as they link the two long bones of the knee together.
What is the definition of a ligament?
A true ligament is a supportive band - made up of strong collagen fibres - that connects one bone to another bone, keeping the joint stable and protecting it from dislocation. A ligament has a certain amount of elastic stretch, but can weaken if too much tension is put onto it. Sometimes ligaments also connect two soft tissue structures, such as the menisci.
What are the cruciate ligaments?
The cruciate ligaments are the central ligament stabilisers of the knee. They are strong bands of ligament tissue that allow the normal knee movement of bending and straightening, as well as some rotation of the knee, but prevent excessive movement or dislocation of the femur and tibia bones. That is, they maintain knee stability - in conjunction with the other stabilisers like the collateral ligaments and the menisci. The ligament you can see arising from the front of the tibia is called the anterior cruciate ligament (ACL), and the one arising behind it is the posterior cruciate ligament (PCL).
The ACL versus the PCL
In this illustration these knee bones have been cut open down the middle of the joint so that you can see the full extent and direction of each cruciate ligament.
It is easy to see that the anterior cruciate ligament (ACL) holds the tibia from sliding forwards. The posterior cruciate ligament (PCL) holds the tibia from sliding backwards. The combined action of the two strong ligaments is to prevent 'antero-posterior' ('forwards-backwards') instability.
The ACL contributes more to the stability of the knee than the posterior cruciate ligament (PCL). For this reason many PCL injuries are missed at first visit to a clinician, and even if they are not reconstructed they contribute much vaguer symptoms than do neglected ACL tears.
Anteroposterior Stability of the Knee during the Stance Phase of Gait after Anterior Cruciate Ligament Deficiency. Chen CH, Li JS, Hosseini A, Gadikota HR, Gill TJ and Li G. Gait Posture. 2012 Mar; 35(3): 467–471.