The definite article “the” tells readers or listeners that its accompanying noun is identifiable; that is, the readers know exactly which specific noun the author is referring to. As long as the definite article is being used in this way to refer to things specifically, it can be used in front of every type of noun and noun phrase (singular count, plural count, and non-count nouns). For example:

There are some exceptions to the rule, and these include:

Geographical terms such as bridge, building, ocean, region, or river

the Confederation Bridge, the Science Museum, the Pacific Ocean, the Northeast, the Thames River

Confederation Bridge, Science Museum, Pacific Ocean, Northeast, Thames River

Plural islands, lakes, and mountains

the Queen Elizabeth Islands, the Great Lakes, the Laurentian Mountains

Queen Elizabeth Islands, Great Lakes, Laurentian Mountains

Reference to a famous proper noun

I was referring to the great Wayne Gretzky.

Someone in my class is indeed named Justin Trudeau, but I was actually referring to the Justin Trudeau–the ex-prime minister of Canada.