Burnout occurs when a portal has been hacked by an Agent four times within four hours. Burnout lasts for four hours after the first hack, preventing the Agent from hacking the portal within that time.
Burnout can be delayed using Multi-hacks, which increase the number of hacks an Agent can make before the portal burns out. Burnout can also be reversed by installing a Heat Sink.
Portal burnout is exclusive to one Agent; that is, when one Agent burns out a portal, the portal shows up as burnt out only for them. Other Agents can still hack a portal that was burnt out by another Agent.