While this term has been used for many years to refer to Pokémon games separate from the core series, my introduction of "supplementary series" means that my definition of "side series" is slightly more strict. I define "side series" games as those that are not compatible with core series games, or if they do have an interactive element, it is not bi-directional. Games contained within this category incidentally often have very different mechanics to the core series and supplementary series.
Defining a full and comprehensive list of Pokémon games that fit into this category is likely an exercise in futility, owing to the sheer amount of Pokémon content released, the fact that some of it is region-specific, and the fact that some is or was available only for a short time with no physical release. Some of the better-known examples (and ones I'm likely to ever talk about here) include the Mystery Dungeon series, the Ranger series, the Snap series, the Pinball series, Trozei, Conquest, and the Pinball series.