General term for a person who fights in battles, from ancient warriors to modern soldiers. Often carries a slightly formal or literary tone compared to 兵士.
He was known as a brave warrior.
Ancient Greek warriors wore heavy armor.
He is known as a warrior for justice.
They fought as warriors until the very end.
武士 refers specifically to a samurai of feudal Japan, whereas 戦士 is a broader term that can describe any warrior or fighter.
戦士 is a native Japanese word evoking a warrior in a traditional or heroic sense, while ファイター is a modern loanword focused on combat sports or personal fighting spirit.
戦士 usually means a soldier or warrior in a military or fantasy setting, while 闘士 stresses fighting for a political, social, or sporting cause.
戦死 (せんし) means 'death in battle' and is written with different kanji, so don't confuse it with 戦士 (せんし), 'warrior/soldier'.
Compound of 戦 (sen, 'war, battle') and 士 (shi, 'person of a certain status, gentleman, warrior'). The exact historical derivation is uncertain, but the combination naturally yields 'warrior' or 'soldier'.