As the achievement detail explains, one needs to complete every Research Project available.
Please note, finishing all of this research requires a sizable amount of these resources:
Various alien corpses
So, it might be smarter not to sell any of those items on the "Gray Market," until all of your research is completed.
**Edit: Also, if you find yourself well into the game and haven't yet caught a live Sectoid or Thin Man, make sure to use the Council Missions to your advantage -- the "Asset Recovery" Council Missions are almost exclusively populated with Sectoids, and most of the other council missions have Thin Men. Too, you may re-load saves just before Council Missions come up, and you will likely get a different council mission option, so you can pick exactly the kind you need.
If you're missing the basic Floater, keep an eye out for "Terror!" missions, as you will occasionally see groups of Floaters, as well as groups of Heavy Floaters, during these missions. End Edit**
Here is a list of ALL 53 Research Projects, as listed exactly in the Research Archives (it looks as though the completed list will be in the same order for each of us, as the list is not in any chronological order of research performed):
New Fighter Craft
Light Plasma Rifle
Fusion Lance (requires obtaining an alien "Fusion Core" from a Battleship)
Guided Fusion Launcher (requires obtaining an alien "Fusion Core" from a Battleship)
Alien Nav Computer
UFO Power Source
Interrogate Sectoid Commander
Interrogate Thin Man
Interrogate Heavy Floater
Interrogate Muton Elite
Sectoid Commander Autopsy
Thin Man Autopsy
Heavy Floater Autopsy
Muton Elite Autopsy
** Note: Having the "Slingshot" DLC will likely give early access to the "Fusion Lance" and "Guided Fusion Launcher" projects, as one of the missions in this DLC package will have you invading a grounded Battleship quite early in your campaign, where you may recover 2 alien "Fusion Cores" (they are recovered automatically, at the end of the mission, provided you haven't blown them up) that are required for these research projects.