Phantasm, n.
Etymology: [L. phantasma. See Phantom, and cf. Fantasm.]
[Spelt also fantasm.]
1. An image formed by the mind, and supposed to be real or material; a shadowy or airy appearance; sometimes, an optical illusion; a phantom; a dream. They be but phantasms or apparitions. Sir W. Raleigh.
2. A mental image or representation of a real object; a fancy; a notion. Cudworth. Figures or little features, of which the description had produced in you no phantasm or expectation. Jer. Taylor.
Shades of meaning
Fernald has no entry of its own for phantasm; he lists it under delusion.
He lists it without comment; the entries above explain its neighbors.