What is it

[CharlieW]

[Rockwood]

It's probably rugose coral. More angles of it and where it was found would be helpful. 

[doushantuo]

Dale is probably correct in assuming this is a rugose (which boils down to "essentially paleozoic" ) coral

Below: classic coral imagery form Milne-Edwards

Out of copyright/freely usable

Edited December 8, 2024 by doushantuo

[Fossildude19]

I agree, this is a worn Rugose coral.

[Rockwood]

It may be a piece from a branching colonial form of rugose. I've seen short segments like this in external mold fossils. I assume it to be the result of cyclical growth. 

[BobWill]

Since we don't know the age maybe we shouldn't rule out one of the Cretaceous "button" corals which can have a similar look. @CharlieW it might be helpful of you tell us where it came from.