from the Fboats list, Ira Heller:

"Re: reachers and code zeros

This is the definition from the ISAF Racing Rules of Sailing.

Part 4 OTHER REQUIREMENTS WHEN RACING Page 54

50.4 Headsails
The difference between a headsail and a spinnaker is that the
mid-girth of a headsail, measured from the mid-points of its luff and
leech, does
not exceed 50% of the length of its foot, and no other intermediate
girth exceeds a percentage similarly proportional to its distance from
the head of the sail. A sail tacked down behind the foremost mast is
not a headsail.

So, by US Sailing and International Sailing, a screacher is a spinnaker.

Side note: The original screacher for Corsair trimarans was
introduced by then Florida Corsair dealer Eric Arens and his
sailmaker Randy Smyth. It debuted at the Ft. Lauderdale - Key West
Race in January, 1993. The intent was to come have a sail to fill
the gap between the jib and the spinnaker yet not have to be rated as
an upwind sail and incur a rating penalty. The beginning of that
race was upwind and we (Pat Hogan's F-27 Hootin' along with me, Rick
White and someone else) were pacing JusTRIte II (Arens, Smyth, Notary
(as I recall) and someone else). But at some point, as we followed
the curve of lower Florida, the wind came somewhat aft to where we
had to barber haul the jib out. That's when JusTRIte II unrolled a
new sail and proceeded to do a horizon job on us. We were to later
learn that they had employed a new, never before seen sail that they
referred to it as a screacher.

Regards, Ira Heller"