From Sega Retro
A perhaps vague term, "Multi-Phase" describes games having multiple "phases" as the player progresses, with more challenges added to the game after objectives are completed. For example, Sega's first multi-phase game, Head On (1979), introduces more enemy cars after all the dots are removed from the screen, forcing players to reassess their gameplay strategy. Earlier games like Space Invaders increase in difficulty by simply changing the speed of the enemies faster - the initial number of invaders is always the same, and the same pattern is always followed.
In some respects "Multi-Phase games" are precursors to games which have different "levels" or "screens" - where gameplay can fundamentally change once tasks are completed. Sega trademarked the term and promoted the idea in North America, but this style of difficulty curve is not thought to have been invented by the company, nor was it exclusive to Sega products. Aguably video games naturally evolved into as this style of game as technology improved, as opposed to being directly influenced by Sega.
List of games
The following lists releases specifically designated by Sega as "multi-phase games".