Here's a good definition according to brewingtechniquies.com:
Lauter grant:
When using a pump to
move wort from the lauter tun to the
kettle, it is easy to pull liquid from
the tun faster than the grain bed
wants it to flow, compacting the bed
and causing a stuck runoff. Aside from
being a large pain, a stuck runoff can
also damage the pump (magnetically
coupled pumps can be damaged if run
dry).
One solution to this problem is to
purchase a speed controller for your
pump, but these are expensive. Another
option is to install a ball valve on
the outlet side of a centrifugal pump
to restrict flow, but ball valves can
be difficult to use for fine-tuning
liquid flow. A lauter grant represents
another possibility.
A lauter grant is simply a vessel that
collects the wort from the lauter tun.
The pump is plumbed directly to the
lauter grant instead of the lauter
tun. Gravity gently pulls the wort
from the lauter tun, thus minimizing
compression of the grain bed. A float
switch can be used to turn the pump on
when the grant is full, thus freeing
the brewer to tend to other things.
My lauter grant is an old plastic
bottling bucket (see photo on page
45). A hose runs from the lauter tun
to the bottom of the grant (to avoid
aeration of the wort) and a pump is
attached to tubing that leaves the
grant through the spigot. This simple
arrangement eliminates the need for an
expensive pump speed controller.
