What is the term for a body of citizen soldiers?

The term for a body of citizen soldiers is "militia." This concept traditionally refers to groups of citizens who are organized and trained for military service, often in times of emergency or to supplement regular military forces. Militias typically comprise individuals who have not made military service their full-time profession but may be called upon to defend their community or country.

The idea of a militia is rooted in the historical context where local communities would form such groups to provide defense and maintain order, particularly before the establishment of standing armies. These citizen soldiers are often seen as a vital component of a nation's defense capability, embodying the principle of civilian involvement in national security.

In contrast, the other options represent different types of military or defense forces. A regular army consists of professional soldiers who serve full-time, civil defense refers to organized efforts to protect civilian populations in emergencies, and volunteer forces typically consist of individuals who choose to serve but may not fit the specific definition of a militia as citizen soldiers.