Esters yog ib chav kawm ntawm cov organic tebchaw tsim los ntawm cov tshuaj tiv thaiv ntawm cov kua qaub (xws li carboxylic acid los yog ib qho inorganic oxoacid) nrog cawv. Cov qis dua -molecular- hnyav esters yog cov kua dej tsis haum xeeb uas muaj cov ntxhiab tsw ntxhiab, qhov siab dua -molecular- hnyav esters muaj nyob hauv cov khoom siv quav ciab lossis cov kua dej khov heev. Ntau qhov siab dua -molecular- hnyav esters yog cov tseem ceeb ntawm cov rog. Hauv kev siv tshuaj lom neeg, "qis" esters feem ntau xa mus rau cov uas muaj cov pa roj carbon tsawg tsawg, thaum "siab dua" esters hais txog cov uas muaj cov pa roj carbon ntau. Qhov kev sib txawv no-siv lawv "qis" piv rau "siab dua"- tshwm sim los ntawm qhov tseeb tias qis dua -molecular- hnyav cov tebchaw, muaj tsawg dua carbon atoms, nthuav tawm cov qauv yooj yim; conversely, siab dua -molecular- hnyav cov tebchaw, nrog lawv cov pa roj carbon atoms ntau dua, tuaj yeem muaj nyob raws li ntau yam isomers uas tshwm sim los ntawm kev sib txawv spatial ntawm lawv cov atoms-cov qauv complexity uas yog qhov "siab dua" lossis siab dua.
Cov mis mos molecular rau esters yog R-COO-R' (qhov twg R yuav yog ib pawg hydrocarbon lossis hydrogen atom, tab sis R' tsis tuaj yeem yog hydrogen atom; txwv tsis pub, pawg ua haujlwm yuav yog pawg carboxyl).
Cov yam ntxwv ua haujlwm pab pawg ntawm ester yog -COO-; cov mis loj rau saturated monoesters yog CnH2nO2 (qhov twg n Ntau dua lossis sib npaug rau 2, thiab n yog tus lej zoo).
