Parasitic describes something that lives off another living thing, taking what it needs while giving nothing helpful in return. A parasite is an organism that survives by feeding on or living inside another creature, called the host. Ticks are parasitic: they latch onto animals and drink their blood. Tapeworms are parasitic: they live in an animal's intestines and steal nutrients from the food their host eats. The host gets weaker while the parasite gets what it needs.

Scientists who study parasites have discovered fascinating and sometimes disturbing relationships. Some parasitic wasps lay eggs inside caterpillars, and when the eggs hatch, the larvae eat the caterpillar from the inside. Mistletoe is a parasitic plant that grows on trees, stealing water and minerals through the bark. Parasites exist throughout nature, from microscopic organisms that cause diseases like malaria to larger creatures like leeches.

The word can also describe people or behaviors that take advantage of others' generosity without contributing anything back. If someone constantly borrows money but never repays it, or always wants help but never offers any, you might hear their behavior described as parasitic. Unlike true friendship, where people support each other, a parasitic relationship only flows one direction.