It was a coin that had no evident circulation, but could have a lot of marks, or almost none. Those differences were conveyed with adjectives such as choice, gem. superb, baggy, etc., and it was up to the buyer to figure out if the coin was what he/she wanted for the asking price.
By itself "uncirculated" was generally a modern MS62 or 63 coin, but much depended on the opinion of the grader. For example, an Unc graded by Jim Ruddy was likely AU, the same description of a coin by Dave Bowers was MS64 or better. In the late 1960s I bought several coins from Dave that were graded "Choice Unc." Years later, PCGS called the worset of them MS-66 and the best MS-68 (seated quarters).