The word "soldier" specifically refers to members of the army who are without officer's commission. In most armies of the world, soldiers who are not officers can work their way through the ranks to obtain a commission.
A soldier who no longer serves in the armed forces is often called a veteran, a term which can also apply to a long-serving or experienced soldier who is still in the army. In the United States soldiers are identified as those serving or once served in the United States Army.

Etymology
The word soldier is derived from an Old French word, itself a derivation of Solidarius, Latin for someone who served in the armed forces for pay, as opposed to warriors in tribal society where every grown man is automatically a member of his clan's fighting force. Solidare in Latin means "to pay"; Roman soldiers were paid in solidi, so-called because they were a new type of solid gold coin brought in after a reform of the Roman money system. The common origin for the words soldier and payment survives not only in French (soldat and solde) but also in other languages, like German (Soldat and Sold), Spanish (soldado and sueldo), Portuguese (soldado and soldo), Dutch (soldaat and soldij), Italian ("soldato" and "soldo") and many other languages.