Berger et al. (2009) attempt to define epigenetics.
"An epigenetic trait is a stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence."Sounds good to me. Just about anything wold be better than the kitchem sink definition proposed by Eva Jablonka [Epigenetics at SEED].
The main examples are "DNA methylation, histone modifications, histone variants, and nucleosome positioning." These are chromosomal alterations that are passed on to daughter cells following cell division by mitosis or meiosis.
Although the Berger et al. don't mention it, these epigenetic signals are all reversible. I still don't find the term useful. It's far more accurate to refer to each of the individual examples by name and the field is "regulation of gene expression."
Berger, S.L., Kouzarides, T., Shiekhattar, R., and Shilatifard, A. (2009) An operational definition of epigenetics. Genes & Dev. 23:781-783. [DOI: 10.1101/gad.1787609]
[Hat Tip: Hopeful Monster]