Depends on whether you are talking about US coins or say coins from Canada. For US it means a coin that is not a Business strike, and not a Proof, but something "special" has been done to it, they may not know what, and often there is no official Mint explanation as to what exactly was done or why. Examples would be the 1927 SP Buffalo nickels, or the special nickels that were in the Thomas Jeffereson and Botanical Gardens coin sets (those depending on the TPG and the time when they were slabbed have been called MS, Matte Proof, SMS, and SP. I think right now PCGS calls them SP and NGC calls them SMS.) The 1894 S dime and 1913 V Nickels, which were originally called MS, and then Proof, I believe are currently transitioning into being called SP. For Canadian coins they had sets that were better quality than their Business strikes, but not up to the quality of what they called Proofs. They were called Specimen sets and the coins in them are Specimen coins. (Frankly I can't tell them from proofs, and they were better quality than the proof coins we were making at the time.)
