ESSENCE

Your essence rating is a measure of your life force.  Every human and
meta-human start off with an essence of 6. As people go through invasive
procedures such as having cyberware installed, their essence goes down, as
part of their living tissue is replaced with wires, chips, and machines.  
People can actually stay alive, though alive is probably not a good term, with
an essence of 0. When essence is this low, they tend to be on the edge of
sanity. For mages and shamans, essence is directly related to their magic
rating. Magic thrives on life, and without life, it is difficult to control
the energies from astral space.  Any time a mage or shaman loses 1 essence,
their magic rating drops 1 point.  For example, if you have an essence of 6
and magic of 6, but decide to buy cyber-eyes, you lose only .20 essence to 
make your essence now 5.80.  The problem is that your magic drops to 5!  There
is no in between according to astral space.  Once your essence drops below
5, your magic would drop another point, and so on.  Non-magic people might
not consider essence valuable, but when you die, you also have the chance to
lose essence; once that drops below 0, you are dead for good.

^WSee Also: ATTRIBUTES, ESSENCE HOLE, ESSENCE INDEX, BIOWARE INDEX, MAGIC LOSS^n