The name Thunder Force comes from the project name the Galaxy Federation uses for the development of their Fire Leo series of small yet powerful spacecraft, which are then used for concentrated offensive operations against the ORN Empire. In Thunder Force V: Perfect System (1998) (VG), the name is used (as "Thunder Force 222") to describe the Earthling strike team that fights against the Guardian computer.
The "Leo" portion of Fire Leo stands for Little, Eternal, Operation as seen in the Thunder Force VI intro video.
Thunder Force was only released on three computers: The Sharp X1, the NEC PC-8801, and the Fujitsu FM-7.