Wikipedia gives an acceptable definition:

In evolutionary biology, group selection refers to the idea that alleles can become fixed or spread in a population because of the benefits they bestow on groups, regardless of the alleles' effect on the fitness of individuals within that group.

• http://en.wikipedia.org/wiki/Group_selection

In the context of biology or ecology, a "population" is defined as being a collection of organisms of the same species:

A population is all the organisms that both belong to the same species and live in the same geographical area.

• http://en.wikipedia.org/wiki/Population

Population: In biology and ecology, a group of organisms of one species, living in a certain area. The organisms are able to interbreed. It also refers to the members of a given species in a community of living things.

• http://www.talktalk.co.uk/reference/encyclopaedia/hutchinson/m0015159.html

population: a group of organisms of the same species inhabiting a given area

• http://wordnetweb.princeton.edu/perl/webwn?s=population

For examples of group selection critics being more sympathetic towards species selection, see Dawkins, T.E.P., page 101 onwards and Mark Ridley's evolution textbook:

• http://www.blackwellpublishing.com/ridley/a-z/Group_selection_.asp

• http://www.blackwellpublishing.com/ridley/a-z/Species_selection.asp

For a different definition, consider:

Group selection is said to occur when the traits of groups that systematically out-reproduce competing groups eventually come to characterize the species.

• http://www.clarku.edu/faculty/nthompson/Publish%20Work/meta4grp.pdf