Semi-octagonal

The spandrils are also highly decorated with tracery mouldings, inclosing shields, &c. and the whole is inclosed in a sort of square frame, with semi-octagonal buttresses.

Source: wiktionary

The Nave (Plate 12) (81 ft. by 18¾ ft.) is of early 14th-century date and has N. and S. arcades of five bays with two-centred arches of two chamfered orders; the piers are octagonal with hollows sunk on the diagonal faces, moulded capitals and chamfered bases; the responds have attached semi-octagonal shafts.

Source: wiktionary

The Octatube nodal joint consists of an octagonal base plate to which were welded two semi-octagonal plates placed at right angles to each other.

Source: wiktionary