In response to a query by 1UP.com, Project Manager Casey Hudson explained the term 'Mass Effect' as follows:
"Inside the game universe, mass effect is a newly-discovered (for humans anyway) physics phenomenon that has properties along the lines of other physics forces such as gravity and electromagnetism. It's what physicists in real life are currently calling "dark energy," as an explanation for the accelerating expansion of the universe -- which has only recently been discovered and flies in the face of the previous notion that the universe's expansion should be slowing down because of gravity."[2]
Thus, mass effect is possibly akin to the theorized fifth force.
In the same article, Hudson explains that certain beings are able to sense and manipulate mass effect much like some real-life creatures (such as sharks and eels) can sense and manipulate electromagnatism. These abilities will be augmentable via implants, and the manipulation of mass effect is referred to as 'Biotics'.