sequence of DNA removed from mature messenger RNA prior to translation. DNA
initially transcribed to messenger RNA consists of coding sequences (exons)
and non-coding sequences (introns); introns are spliced out of the messenger
RNA prior to translation, leaving only the exons to ultimately encode the
amino acid product.
Definition from: GeneReviews
from the University of Washington and the National Center for Biotechnology Information
An intron is a portion of a gene that does not code for amino acids. In the cells of plants and animals, most gene sequences are broken up by one or more introns. The parts of the gene sequence that are expressed in the protein are called exons, because they are expressed, while the parts of the gene sequence that are not expressed in the protein are called introns, because they come in between the exons.