Chemical modifications to DNA and histone proteins form a complex regulatory network that modulates chromatin structure and genome function.
The epigenome refers to the complete description of these potentially heritable changes across the genome.
The composition of the epigenome within a given cell is a function of genetic determinants, lineage, and environment.
References
Bernstein BE, Meissner A, Lander ES. The mammalian epigenome. Cell. 2007 Feb 23;128(4):669-81. PMID: 17320505