Half- or Half is a prefix attached to the offspring of races sharing a mixed ancestry. These are often called half-breeds (but this is considered derogatory, though can be used as a technical term). In the RPG, the term 'half' does not refer to percentages, but rather is just indication of a being's mixed-ancestry no matter how far removed from original source. Thus, for example the offspring of a half-elf who coupled with a human, would still be considered half elf. This rule insures that in most circumstances that those of mixed ancestry are considered part of a wider race-category such as half-orc, half-elf, half-ogre, half-human (or share a combination of categories such as Half-orc half-ogre), despite how removed they are from their original racial ancestors.
Another example is Rexxar who is considered to be both half-orc and half-ogre, though his father is also half-ogre. Though percentage wise that would make Rexxar approximately 3/4ths orc and quarter-ogre.
In rare cases, an individual may be so far removed from one of their ancestral heritage, that they may be considered a full member of their parent's race. For example, Commodore Jessi Falrevere may have a hint of dwarven blood flowing in her veins. This would suggest, that one of her parents was distantly related to dwarfs, but had enough human blood to be considered human, rather than a hypothetical 'half-dwarf'. This would suggest that although one of her ancestors may have been 'half-dwarf' over the generations as her ancestors mated back into human race, she herself was born human.