Blooming
The spilling of charge from a saturated photosite into its neighbours, spreading a bright highlight beyond its true boundaries.
In depth
Once a well is full, excess charge has to go somewhere, and without protective circuitry it leaks sideways. CCDs suffered badly, producing vertical streaks from bright points. Modern CMOS sensors include anti-blooming drains that route the excess away, so the artefact is now rare outside extreme conditions.
Examples in practice
An early digital camera pointed at a street lamp shows a bright vertical spike extending far beyond the lamp itself.