A degenerative brain disorder causing confusion and delusion in old age, which Picard has in one possible future and whose warning signs shadow him for the rest of his life.
Irumodic Syndrome is first named in All Good Things, the finale of The Next Generation. In the future that Q shows him, the retired Picard has advanced Irumodic Syndrome, and his old shipmates doubt his account of an anomaly because of it. Back in the present Crusher finds a small defect in his parietal lobe that could one day lead to the condition.
The series Star Trek: Picard returns to that defect. Its progress leads to his death on Coppelius at the end of the first season, after which his mind is transferred into a synthetic body.
The third season revises the diagnosis: what doctors had read as Irumodic Syndrome was a biological alteration left by the Borg, which Picard had unknowingly passed on to his son Jack Crusher.