Skill

Skills are a civilization knowledge that can be lost over generations. Skills have 11 levels of which all civilizations have access to 10. Skills must be used by a civilization to improve their level, this is initially +1 skill level per generation however civilizations that specialize in that skill type can improve it up to twice a generation. If a skill is not used it loses a skill level every generation but once learned the skill levels can be recovered with an additional level per generation. Skill levels are lost slower with higher education levels.

There are two skill levels active skill level, the level you are currently at. And passive skill level, the highest level you have achieved.

Advantages
Skill levels reduce research points needed for similar technologies, they can reduce cost and improve the efficiency of the trade that the skill is used for. For instance the cost of equipping armor may be reduced by 1% per skill level and the efficiency improved by 0.5% per skill level.