These computer and video games closely resemble other, previously published games. This was common during the early 8-bit computing era when legal action was rare. The themes are usually successful and iconic; the myriad of Space Invaders or Frogger clones, for example.
In later gaming history, the term 'clone' often refers to a style of play that becomes a genre of its own. A Doom clone or a GTA clone would not necessarily be an almost carbon-copy of those game.