What does MS66 mean in silver dollars?

In coin grading terminology "MS" stands for "mint state" or the state a coin is in when minted and uncirculated. Showing no signs of wear or indication of circulation. MS is interchangeable with "uncirculated". Although MS coins have no wear from circulation they may have varying imperfections from the minting process. To allow for that, the "MS" coins are graded on a scale beginning with 60 and going up to 70 (this is the highest number, which indicates supposed "perfection"). So MS66 would mean a "mint state" or uncirculated coin graded 66 out of a possible 70 on the grading scale. A very nice coin.