What is tentsion?

Answer 1

When you stretch something tight. New anchor chains are stretched or have tention applied to them to near breaking point befor being used on a ship. Tention can be used on a spring just prior to its loosing it's elasticity. Almost anything can have tention applied to it as it is a force.

As a point of interest, still water has tention applied to it as demonstrated by the Water Spiders which can walk on its surface.