Elements

From the introduction text:

Welcome to Icar, the free science fiction roleplaying game. This is the Elements, the core rule book. In here, you’ll find all the mechanics of the universe - how everything works. You find out how to make a character, with which you play, and how this character can perform actions in the world.
[..]
This is not the only book you need to play Icar, this book contains only the core rules. The core rules are separated from the settings to allow you to choose the setting that would suit you best.

designer's note:
Since 2012 the Elements has been merged with the other books to form a complete ruleset in one book. The tag "Elements" has been dropped and it is simply called Icar.