A definite article comes before a singular noun that is used as a representative of an entire class of things. This is especially common with the names of animals, trees, inventions, musical instruments, and parts of the body.
The tiger is the largest cat.
My favorite tree is the oak.
The Wright Bothers invented the airplane.
The oboe is a woodwind instrument.
The heart pumps blood.
A definite article is used before expressions with an ordinal number.
No article is used before expressions with cardinal numbers.
the fourth chapter
the seventh volume
A definite article is used before decades and centuries.
the twenty-first century