aesthetic |es the tik|
 ascetic | ə se tik|

These words have similar pronunciations and spellings, but quite different meanings.  The word “aesthetic” –with the “th” sound in the middle–has to do with beauty, so if you’re talking about beauty or the arts, “aesthetic” is probably the word you’d want to use rather than “ascetic.”  “Ascetic” refers to self-denial, self-discipline, and austerity.  It’s a word often used when referring to the monastic life or something similar to it.

aesthetic:  of or relating to beauty or to the study of beauty and its forms.
Her aesthetic sensibility was offended by the gaudy decor of the banquet hall.
Each culture has its own aesthetic traditions.

ascetic:  rigorously simple and disciplined; austere; self-denying.
The nuns lead an ascetic life in the convent.
It was necessary to adopt an ascetic way of life during the war.
Their new stepfather imposed an ascetic regimen on the family.

It’s also easy to confuse “aesthetic” or “ascetic” with “acetic,” which refers to acid or vinegar.  Besides the difference in meaning, the pronunciation of “acetic” has the “e” vowel sounding like the “e” in “bee,” whereas the “e” sound in the other two is like the “e” in “pet.”