Unachievable
Part of speech: adjective
Definition: That cannot be achieved (or only with great difficulty)
Unattainable
Part of speech: adjective
Definition: Impossible to attain or reach; inaccessible, unobtainable or unapproachable; unable to accomplish; beyond limit.
Example sentence: Aim at perfection in everything, though in most things it is unattainable. However, they who aim at it, and persevere, will come much nearer to it than those whose laziness and despondency make them give it up as unattainable.