During an attack, a lateral movement is the movement of the adversary to other user accounts (e.g., through host privilege escalation or leaked credentials). This movement could be to another host or to the same host.
Disambiguation
Lateral movement is often confused with pivoting. While in lateral movement the attacker can move to another account but stay on the same host, pivoting is strictly movement between hosts. That said, many pivoting methods can be used for lateral movement.