Apart from the potential long-term effects of suffering two concussions in a short period of time, there’s a much more important short-term concern that arises from the possibility of a player suffering a concussion before he has recovered from a prior one.

The phenomenon is known as “second impact syndrome.” That term was trending on Thursday night, after Dolphins quarterback Tua Tagovailoa suffered an obvious head injury against the Bengals. If he had suffered a concussion on Sunday that had not yet fully resolved itself, he could have experienced second impact syndrome.

Here’s an explanation of the condition, from the National Library of Medicine: “The generally accepted cause relates to sustaining a second concussion before the brain has a chance to recover from the initial insult fully. The athlete will rapidly develop altered mental status and a loss of consciousness within seconds to minutes of the second hit resulting in catastrophic neurological injury. The catastrophic injury results from the dysfunctional cerebral blood flow autoregulation leading to an increase in intracranial pressure. The pressure rapidly develops and eventually results in brain herniation. The herniation may occur either medially across the falx cerebri or inferiorly through the foramen magnum, resulting in brain stem injury and rapid deterioration and leading to death within 2 to 5 minutes.”

The term was coined in 1984, after a football player suffered a head injury, returned to play again four days later, suffered another head injury, collapsed, and died.

The NFL has never experienced a player dying on the field from a head injury. In August 2001, Korey Stringer died due to heat exhaustion at training camp. In October 1971, Chuck Hughes died during a game, from a heart ailment.