It’s the Latin word for a funnel, derived from infundere, “to pour”, plus the ending –bulum which formed the names of instruments.
In English, it turns up in various anatomical contexts for something funnel-shaped. For example, in the human body it describes the outermost section of the fallopian tubes, a structure in the cochlea of the ear, and a formation in the brain close to the pituitary, among others. Here's another sense:
The deer genus name Odocoileus means “hollow tooth” and refers to the hollow pits (called infundibula) in the chewing surface of some of the teeth of these animals.
Bedford Gazette (Pennsylvania), 25 Nov. 2003.
Science-fiction fans will probably have come across the splendid chronosynclastic infundibulum, invented by Kurt Vonnegut in The Sirens of Titan. He explained this, perhaps less than helpfully, as being “those places ... where all the different kinds of truths fit together”.
The plural is infundibula and the adjective these days is usually infundibular, but others such as infundibuliform and infundibulate have also been used from time to time.