An introduction refers to a beginning — maybe presenting someone new to a group, or inserting a new idea into a project. In a piece of writing, the introduction makes clear to the reader the text's purpose.
The noun introduction comes from the Latin verb introducere, meaning "to lead in." This “leading in” might be the formal introduction of a speaker, or the speaker's own introduction to his remarks. Or it might be the insertion of a new and different idea, as in British politician Lord Hailsham's remark, "The introduction of religious passion into politics is the end of honest politics, and the introduction of politics into religion is the prostitution of true religion."