a definition of "living being"

dear friends,
my name is Emiliano and I need a help about an issue:
in order to find a suitable definition of "living being", it came back
to my mind something that I heard when I was at school, years ago. The
memory is quite blurred, now, but it sounded something like
c.i.t.r.o.e.n.s. (an acronymus meaning the list of features that
define a living organismus).
If somebody can help me with that, it would be a great help for me.
My best regards,
Emiliano