A lead-in sentence is a sentence that is used as an introduction or opening to a larger thought. A lead-in sentence can be used for a paragraph or a multipage paper. In longer papers, however, it is more common for the entire opening paragraph to be used as the lead in.
Common types of lead-in sentences are famous quotes, definitions or questions. Most importantly, the job of the lead-in sentence is to get the reader's attention. Something that piques the reader's interest and leaves the person hungry for more information is ideal. For academic papers, it is best to have a more formal lead-in that does not rely on too much emotion or drama.