Movement at the ankle that brings the toes up toward the shin (dorsiflexion) or points them away (plantar flexion).
Plantar flexion is a calf raise and the push-off of every stride. Dorsiflexion is the opposite, and it is the one that limits people in the gym.
A squat needs the shin to travel forward over the foot, which is dorsiflexion. A client who lacks it lifts their heels, or the knees stop travelling and the hips take over and the squat turns into a good morning. Test it with a knee-to-wall: barefoot, toes 10 cm from a wall, drive the knee to touch the wall with the heel down. Under about 10 cm on one side is a restriction worth working on, and a side-to-side difference of more than 2 cm matters more than the absolute number.
Two fixes work: calf and soleus stretching held 45 seconds, and heel elevation. Putting the client on 15 mm lifting shoes or a small wedge changes the squat immediately while the mobility work catches up over the next month.