Burnout occurs when a portal has been hacked by an Agent four times within four hours. (Not to be confused with Cooldown, which is typically 5 minutes) Burnout lasts for four hours after the first hack, preventing the Agent from hacking the portal within that time. When a player attempts to hack a burnt out portal, an alert will pop up, reading "Portal burned out! It may take significant time for the Portal to reset."
Burnout can be delayed using Multi-hacks, which increase the number of hacks an Agent can make before the portal burns out. Installing a Heat Sink will immediately reset the burnout counter for the Agent who installed it.
Like cooldown, burnout is per agent; hacking never affects other agents' gameplay. Other agents can still hack a portal that burnt out for you.