In the past, Scotch was variously spelled whiskey and whisky; in the last century the latter spelling became standardized.
By law all whisky made in Scotland must follow certain rules, and is then defined as Scotch. To simplify, Scotch must be (a) made from grains, (b) aged in oak casks, (c) for no less than three years, and (d) distilled at an ABV less than 94.8%, and (e) must be completely made in Scotland.
Note that it is actually illegal to produce whisky in Scotland that doesn’t comply with Scotch Whisky Regulations 2009 (Citation 2009, No. 2890)
“the only whisky that may be manufactured in Scotland is Scotch whisky. The definition of manufacture is “keeping for the purpose of maturation; and keeping, or using, for the purpose of blending, except for domestic blending for domestic consumption”. This provision prevents the existence of two “grades” of whisky originating from Scotland, one “Scotch whisky” and the other, a “whisky – product of Scotland” that complies with the generic EU standard for whisky. ” – Wikipedia, Scotch Whisky
According to the Scotch Whisky Association these are the official categories
Single, Malt Scotch Whisky
A Scotch whisky distilled at a single distillery (i) from water and malted barley without the addition of any other cereals, and (ii) by batch distillation in pot stills.
100% malted barley only. Many Americans think of this as the classic Scotch, although in fact, Scots traditionally drank Scotch blends which include this, but also some of the other forms, below.
Single, Grain Scotch Whisky
A Scotch Whisky distilled at a single distillery (i) from water and malted barley with or without whole grains of other malted or unmalted cereals, and (ii) which does not comply with the definition of Single Malt Scotch Whisky.
The mash bill may include unmalted barley, or quite often, wheat. Other permissible grains include corn, rye, triticale or spelt. It usually has at least 5% malted barley to begin the chemical process of saccharification -producing fermentable sugars. One may also use malted corn or rye, but process is more complicated.
Blended Scotch Whisky
A blend of one or more Single Malt Scotch Whiskies WITH one or more Single Grain Scotch Whiskies.
So one component is a pure malted barley whisky, and the other whisky had a mash bill of some grain other than just malted barley.
Blended Malt Scotch Whisky
A blend of Single Malt Scotch Whiskies, which have been distilled at more than one distillery.
The mash bill thus would consist of malted barley whiskys only, although there might be several such whiskys.
Blended Grain Scotch Whisky
A blend of Single Grain Scotch Whiskies, which have been distilled at more than one distillery.
Here, all of the whiskys had mash bills other than just barley. They could contain almost no barley – it could be an almost pure rye, corn or wheat base scotch – thus like some American whiskeys. However, that latter case is generally not produced in Scotland.
Definitions from Scotch Whisky Association, Regulations 2009