from The Century Dictionary.
- noun In heraldry, a figure having eight lobes or eight subdivisions, like separate leaflets. It is used as the mark of cadency for the ninth son.
from Wiktionary, Creative Commons Attribution/Share-Alike License
Above, far back in the clerestory arches, are octofoil windows with sills of over-lapping courses, which incline forward to the string course above the triforium.
The windows in the gable consist of two lesser windows, two-light, with quatrefoil heads, beneath a large octofoil, the whole grouped with blank panels at the side, beneath a cinquefoil moulding.