Differences in orphan definition have program and policy implications. It is thus, very important that researchers, epidemiologists and policy-makers explicitly state their understanding and usage of the term "orphan". Not only does it vary if we approach it from an epidemiological or a legal point of view, but the ordinary language usage varies among people of different cultures and ethnic groups.
Michaels & Levine (1992) of the Orphan Project use the term "orphan" to describe children who have lost one or both parents. They indicate that a majority of the women with AIDS are single mothers and, thus, their children will lose their only parent. This rationale may not be accurate in all contexts and geographic locations, though.
From Greek 
(orphanos), and Late Latin orphanus, the American Heritage Dictionary of the English Language defines orphan as a "child whose parents are dead". According to the on-line Encyclopaedia Britannica and the Merriam-Webster Dictionary an orphan is: (1) a child deprived by death of one or usually both parents; (2) a young animal that has lost its mother, and (3) one deprived of some protection or advantage-e.g., orphans of the storm.
These definitions contain several important elements and distinctions: on one hand, there is a child who may have lost one or both parents; on the other, there is an emphasis on maternal orphanhood, as it leaves the young animal (also true of infants) in a particularly vulnerable situation; finally, there is a figurative use of the word, which puts on the same level parentless children and people who are alone, solitary; abandoned, cast-off, forsaken, lost; disregarded, ignored, neglected, slighted.
The definition of 'AIDS orphan' used by UNAIDS, WHO and UNICEF is of a child who loses his/her mother to AIDS before reaching the age of 15 years. Some of these children have also lost, or will later loose, their father to AIDS. A child whose father dies typically experiences serious psychological, emotional, social and economic loss. However, because reliable data on the number of paternal orphans are not available in many countries, the orphan statistics used by UNAIDS and UNICEF do not include children who have lost only their fathers.
Overall, there are variations in the age up to which children are considered orphans (14, 15, 18 or 21 years old) and the patterns of parental death (both parents die, either parent died or death of mother only). As I said before, it is important that we state our definition. Here, we use UNAIDS's understanding of the term.