This is a tooth from the Late Cretaceous dinosaur known as Triceratops. These dinosaurs had a series of interlocking teeth (called a dental battery) that they used to feed on coarse vegetation like conifers, ferns and cycads. When a tooth was worn, it would be shed and replaced by another. This is an example of a shed tooth. Normally a Triceratops tooth will have 2 roots, but this one has been eroded and damaged over time.
The Children's Museum of Indianapolis
Creative Commons (CC By-SA 3.0);
Paleontology -- Cretaceous
Further information on this record can be found at its source.