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 figure of speech in which a part signifies the whole (fifty masts for fifty ships) or the whole signifies the part (days for life, as in He lived his days in Canada). Also when the name of the material stands for the thing itself (pigskin for football)