Spirit

From the helpfile:

Spirit Points:

Spirit points are a direct reflectant of how much spirit you can use before
becoming exhausted. Spirit points are extremely important and are only used
by invokers.  Spirit regeneration is directly affected by willpower. The
greater the will, the faster the spirit regen.  Additionally, spirit regen 
rates include a 52 point regeneration per phase, plus the percent regenerated 
per phase.

For more information, see: help invoke, help cast

Score Sheet Topic

Score Sheet Topic: Spirit

This field represents the current and maximum spirit you have. Spirit is used
mostly for invoking spells but can also be used for certain other aspects such
as magical shields.

For more information, see: help spirit