The addition of a chemical group (methyl) to DNA that epigenentically alters gene expression.
Enzymatic addition of methyl (CH3) group to DNA which causes inactivation of that region. Usually CpG nucleotide pairs are target for this addition.
Methylation of cytosine (usually in CG stretches) -- sign for transcription factors to activate the gene and thus produce a protein. It is generally understood that genes that are available for transcription are sometimes less heavily methylated than the same genes in cells in which they are never expressed. It also refers to the addition of a methyl group to a chemical compound or macromolecule.