The phrase "air built" can refer to something that seems to exist only in one's imagination or something that is lacking in substance and depth. A few synonymous phrases for "air built" include "illusory," "ethereal," and "ephemeral." "Illusory" suggests something that is not real, but only appears to be so. "Ethereal" describes something that is delicate and otherworldly. "Ephemeral" refers to something that is fleeting and short-lived. Other synonyms for "air built" include "fantastical," "imaginary," "airy," and "dreamlike." All these words indicate something that is not very substantial or grounded in reality.