This is the most consistently misread term in spirits, and the misreading is understandable: "single" naturally suggests one of something, and the one thing it does not mean is one cask.
What each word is doing
Single: the product of one distillery. Malt: made from malted barley and nothing else. In Scotland it must additionally be distilled in pot stills and matured at least three years in oak casks in Scotland; comparable definitions exist in Ireland, Japan and elsewhere, and they are not identical.
Single cask is a different thing
A bottling drawn from one named cask is a single cask, it is far rarer, and it says so on the label — usually with a cask number and an outturn. If a bottle does not claim it, it is a blend of casks from that distillery, which is normal and is how a house style is held steady.
The neighbouring categories
A blended Scotch is malt whisky and grain whisky from more than one distillery. A blended malt is malt whisky only, from more than one distillery. A single grain is grain whisky from one distillery. Each of those is a precise term and each is routinely used loosely.