The smallest or weakest animal in a group, especially in a litter of animals.
People might feel sorry for the runt and want to take care of it.
Examples:
The runt of the litter is very small.
The runt puppy is the smallest one.
The runt piglet is the weakest.
The runt kitten is so tiny and cute!
The runt of the litter needs extra care.
The runt of the litter is always the smallest.