The Battle Rifle is a far from universal term for an assault rifle that fires "full calibre" ammunition - that is something big and nasty like 7.62NATO, 7.92 Mauser or 7.62 Russian - rather than a reduced power round like 5.56NATO, 5.54WarPac or even 7.62 WarPac or 7.92 Mauser Kurtz.
These weapons have the advantage that they retain the range and hitting power of an old fashioned rifle, but against that they retain an old fashioned size and weight as well, not to mention being effectively uncontrollable on full automatic. They are usually older weapons, adopted in the post WW2 era by nations not yet ready to accept a less powerful individual weapon, but recent experiences in Afghanistan have lead several nations to re-introduce "full calibre" rifles for at least some of the men in an infantry section to benefit from the increased range.
Examples of the transitional rifle include the H&K G3, the FN-FAL and the original M14 and their more modern descendants include the fully automatic version of the L129, the HK417 and the FN SCAR-17.
As previously noted the term "battle rifle" is not universal, and occasionally gets used for other things or ignored completely.