Noun
A
transparent
plate
marked
with
a
crosshair
,
grid
,
or
scale
which
is
used
in
an
optical
instrument
,
etc
.,
to
aid
in
alignment
,
counting
,
or
measurement
of
objects
being
observed
;
also
,
the
crosshair
,
grid
,
or
scale
on
such
a
plate
.