The following information is from the book "Breeding Merinos", by C.H.S. Dolling.
In rams, ewes, or wethers.
Depression: A concave depression in bone of skull on the site.
Knob: Bony knob on the site. Ranges from being so small as to be not obvious during a quick handling of the site, to 1/2 " or more in height.
Scur: A knob capped with horny material which is not completely removable by rubbing with fingers. May range from less than 0.1" in length, to several inches.
Horn: A horny growth firmly attached to its horn core growing from the frontal bone. Generally serrated, pointed, with the two horns symmetrical.
Knob in Depression: A knob set in a depression.
Scur in Depression: A scur set in a depression.
In rams only:
Short scur: A stocky scur, less than approximately 3" in length in the adult. No depression present.
Long scur: A scur approximately 4-10 or 12 inches in length.
Aberrant horn: Typically a well-grown horn, firm, serrated with pointed tips, smaller than a true horn. A miniature true horn.
True Horn: Normal, large, coiled horns.
More useful definitions:
Full-poll: A sheep with the horn genotype PP.
Half-poll: A sheep with the horn genotype Pp.
Horned: A sheep with the horn genotype pp. (There is no half-horned.)
Single poll: A sheep with one polled, and one horned parent. A single-poll must be Pp.
Double poll: A sheep with two polled (hornless) parents. May be a half poll (Pp), or a full-poll (PP).