I was wondering whether a fighter or a combatant in a guerrilla army could be defined as being a soldier or a partisan; when looking up Guerrilla Warfare in wikipedia I found that guerillas used in the plural, refers only to the members of this military force.
The term means "little war" in Spanish, and the word, guerrilla
(Spanish pronunciation: [geˈriʎa]), has been used to describe the
concept since the 18th century, and perhaps earlier. In correct
Spanish usage, a person who is a member of a guerrilla is a
guerrillero ([geriˈʎeɾo]) if male, or a guerrillera if female.
The term "guerrilla" was used in English as early as 1809, to describe
the fighters (e.g., "The town was taken by the guerrillas"), and also
(as in Spanish) to denote a group or band of such fighters. However,
in most languages guerrilla still denotes the specific style of
warfare. The use of the diminutive evokes the differences in number,
scale, and scope between the guerrilla army and the formal,
professional army of the state.
Looking further, I found the term "guerrilla fighters" received 64,900 results on Google books, whereas guerrillas 1860 received 44,800 results results and guerrillas 1970 received 125,000 results. How many of the latter are actually reference books tracing the history of guerrillas and guerrilla warfare and not the actual guerrillas fighting between 1960 and 1970s is open for debate.
It could be that nowadays, the term geurilla fighters has superseded guerrillas in usage and this might, in part, account for the disparity in frequency.