1. Any tubular extension of the head or mouth parts; especially, one used for feeding.

An elephant's trunk, perhaps the best known example, is a long muscular elongation of the upper lip and nose. It is used for transferring food to the mouth, and for sucking up water, which is then squirted into the mouth.

The end of the trunk has a pair of nostrils and also finger-like lobes, which can pick up small objects.

2. The slender tubular feeding and sucking structure of some insects (moths or butterflies) and worms.
3. In general, a proboscis is an elongated appendage of a living organism.

The most common usage is to refer to the tubular feeding and sucking organ of certain invertebrates like insects, worms, and mollusks.

4. The term is used for primate organs as well: an elongated human nose, especially a prominent one, is sometimes called a proboscis (as a joke) and the Proboscis Monkey is named for its enormous nose.