Many times we find in old genealogical documents or religious records the sentence "legitimate son born from legitimate marriage". This sentence is clear and relates to children that were conceived when they parents were married.
However, there is some confusion on the terminology used in old Spanish documents for the children that do not fit this situation. There are different terms used to name these extramarital children (hijos extramatrimoniales) that describe different situations.
When the documents record a "hijo natural" (natural children) they refer to children whose parents were not married but they were free to do it if they wanted to, because both were single. The "hijos naturales" could be recognized, legitimated by their parents and could carry the father's surname.
They should not be confused with the documents that record illegitimate children (hijos ilegítimos). When these children were conceived their parents were not free to marry, and the old documents sometimes provide additional details about these children, providing three categories:
1) Adulterine (adulterino) or sometimes known as bastard (bastardo): one of the parents is married to a different person.
2) Sacrilegious (sacrílego): father was a priest or mother was a nun
3) Incestuous (incestuoso): parents belonged to the same family on a degree of consanguinity not allowed by the church (1st to 3rd degree of consanguinity)
Finally, we can also clarify the terminology for brothers and sisters. Brothers and sisters that share only one progenitor are half-brothers (medio hermanos). On the contrary stepbrothers (hermanastros) are children of each one of the spouses that do not have any degree of consanguinity. Interestingly, stepbrothers /stepsisters can get married as they are family by affinity but they do not share a common ancestor.