phantasia
A phantasm (an impression received through the senses) or the faculty of receiving or representing these impressions.
Phantasia, then, is the animal's awareness of some object or state of affairs, which may well prove to be an object of desire. [...] I am thirsty; I have (in the absence of the object) a phantasia of drink. [...] Phantasia can also be used to account for delusion in practical cases: "Poor dog, he saw that as water (or as drink) when it was really ammonia solution."
Something imaginary; a fantasy.
dated
But even here suppuration does not always stop, reaching often the fore-arm, and in such cases even life is endangered. Unhealthy granulations, thrombosis of veins, septicæmia and pyæmia cause death. This picture is not merely a phantasia, but exists in reality, and I myself had occasion to witness two cases of this kind in the surgical wards of Berlin; [...]
Where it comes from
Borrowed from Latin phantasia (“fancy, fantasy; imagination”), and from its etymon Ancient Greek φᾰντᾰσῐ́ᾱ (phăntăsĭ́ā, “appearance, look; display, presentation; pageantry, pomp; impression, perception; image”), from φᾰ́ντᾰσῐς (phắntăsĭs) + -ῐ́ᾱ (-ĭ́ā, suffix forming feminine abstract nouns). Φᾰ́ντᾰσῐς (Phắntăsĭs) is derived from φᾰντᾰ́ζω (phăntắzō, “to make visible, show; to become visible, appear; to imagine”), from φαίνω (phaínō, “to appear; to reveal; to shine”), ultimately from Proto-Indo-European *bʰeh₂- (“to shine”). The English word is a doublet of fancy, fantasia, fantasy, and phantasy.
Related words
Adapted from the Wiktionary entry phantasia (CC BY-SA 4.0).