What is a session in regards to security?

For purposes of encryption, a session on a PtP system (Backhaul) starts when the Backhaul timing slave registers to the Backhaul timing master, and thereafter a new session is forced every 24 hours.
On a PtMP system (AP and SMs) a new session occurs for an SM when it registers to an AP and then remains until interference, loss of sync, or rebooting of the AP or SM forces a new session.