1. a traffic accident involving multiple vehicles that collide in a chain reaction
2. an accumulation or buildup of things in a disorderly heap

Examples

• The icy highway caused a massive pileup involving twelve cars.
• Emergency crews worked for hours to clear the pileup on the interstate.
• There was a pileup of dirty dishes in the sink.
• The pileup of paperwork on her desk was overwhelming.
• Snow caused a dangerous pileup during rush hour.
• He organized the pileup of books scattered across the floor.