(the early overall composition of the silicate portion of Earth)
Bulk silicate earth (BSE) refers to the
early composition of the silicate portion of the Earth.
Studies are carried out to determine it, using
studies of the present Earth composition and the composition of
meteorites presumed to date from the time of Earth's formation.
Models of the Earth's BSE come into play when comparing
Earth and its formation to extra-solar planet observation
and models.
I've also seen the abbreviation BSM used for bulk silicate Mars.