Litter consists of waste products that have been discarded incorrectly, without consent, at an unsuitable location. The word litter can also be used as a verb; to litter means to drop and leave objects, often man-made, such as aluminum cans, paper cups, food wrappers, cardboard boxes or plastic bottles on the ground, and leave them there indefinitely or for other people to dispose of as opposed to disposing of them correctly.

Large and hazardous items of rubbish such as tires, electrical appliances, electronics, batteries and large industrial containers are sometimes dumped in isolated locations, such as national forests and other public lands.

It is a type of human impact on the environment and remains a serious environmental problem in many countries. Litter can exist in the environment for long periods of time before decomposition and be transported over large distances into the world's oceans. Litter can affect the quality of life.

Cigarette butts are the most littered item in the world, with 4.5 trillion discarded each year. Estimates on the required time for cigarette butts to break down vary, ranging from 5 to 400 years for complete degradation.