M78 is the name of a mutation on the Y chromosome.
It is used as a UEP to identify the Y chromosomal clade named "E-M78", or, in phylogenetic terms "E1b1b1a1".
There will probably be significantly more information posted at the article named "E-M78". This article is only about the mutation.
Before Trombetta et al. (2011) it was E1b1b1a.