-ac Also -acal.
Forming adjectives and nouns.
[Latin -acus or French -aque from Greek -akos.]
All words ending in -ac from this source include an i from the original Latin or Greek root, so making it in effect -iac.
Some are adjectives: cardiac (Greek kardia, heart), of the heart; iliac (Latin ilia, entrails), relating to the ilium, a bone in the pelvis, or to the nearby regions of the lower body. Others are nouns: aphrodisiac, a food, drink, or drug that stimulates sexual desire; zodiac, the belt of the heavens that includes the apparent paths of the sun, moon, and principal planets. Some can be both noun and adjective: amnesiac, hypochondriac, insomniac.
Nouns often have linked adjectives in -al (see -al1), as in zodiacal and ammoniacal. In a few cases, adjectives in both forms co-exist, as with paradisiac and paradisiacal, or heliac and heliacal (though in both these cases the form in -acal is much more common).
Some words in -ac have related forms in -ic: demoniac and demonic, haemophiliac and haemophilic, maniac and manic. In each of these cases the form in -ic is an adjective, irrespective of the role of the form in -ac. An exception is amnesiac, in which amnesic can be both adjective and noun.