Chemistry, Physics . the smallest physical unit of an element or compound, consisting of one or more like atoms in an element and two or more different atoms in a compound.

Chemistry . a quantity of a substance, the weight of which, measured in any chosen unit, is numerically equal to the molecular weight; gram molecule.

any very small particle.

noun

bit , fragment , iota , jot , minim , mite , modicum , mote , ounce , particle , ray , speck , unitnotes:atom comes from greek atomos \'uncut , indivisible\' - based on a- \'not\' and temneir \'to cut\' and it is the smallest unit of matter that retains the characteristics of an element; a molecule is the smallest unit of matter that retains the characteristics of a compound , crumb , dab , dash , dot , dram , drop , grain , ort , scrap , scruple , shred , smidgen , tittle , trifle , whit , atom , ion , monad , part , unit