Intron

An intron is a non-coding sequence in a gene.
It is any nucleotide sequence within a gene that is removed by RNA splicing to get the final RNA product of a gene. The term "intron" refers to both the DNA sequence within a gene, and the corresponding sequence in RNA transcripts. 
Sequences of coding DNA which are joined together in the final RNA after RNA splicing are exons. They code for amino acids in the final polypeptide.
Introns are in the genes of most organisms and many viruses. They can be in a wide range of genes, including those that generate proteins, ribosomal RNA (rRNA), and transfer RNA (tRNA). RNA splicing takes place after transcription and before translation.
The discovery of introns led to the Nobel Prize in Physiology or Medicine in 1993 for Phillip Sharp and Richard Roberts. The term "intron" was introduced by American biochemist Walter Gilbert.
Biological meaning.
There are many unanswered questions about introns. It is unclear whether introns serve some specific function, or whether they are selfish DNA which reproduces itself as a parasite.
Recent studies of entire eukaryotic genomes have now shown that the lengths and density (introns/gene) of introns varies considerably between related species. There are four or five different kinds of intron. Some introns represent mobile genetic elements (transposons).
Alternative splicing of introns within a gene allows a variety of protein isoforms from a single gene. Thus multiple related proteins can be generated from a single gene and a single precursor mRNA transcript. The control of alternative RNA splicing is performed by complex network of signalling molecules. In humans, ~95% of genes with more than one exon are alternatively spliced.