Measuring instantaneous rates is the most direct way of determining the rate law of a reaction, but it is not always convenient, and it may not be possible to do so with precision.
piece (pÄs) n. 1. A thing considered as a unit or an element of a larger thing, quantity, or class; a portion: a piece of string. 2. A portion or part that has been separated from a whole: a piece of pie. 3. An object that is one member of a group or class: a piece of furniture.
a separate or limited portion or quantity of something: a piece of land; a piece of chocolate. a quantity of some substance or material forming a single mass or body: a nice piece of lumber. a more or less definite portion or quantity of a whole: to cut a blueberry pie into six pieces.