It is clear that a cleanroom is a room that is clean. However, a cleanroom now has a special meaning and it is defined in the International Organization for Standarization (ISO) standard 14644-1 as:
A room in which the concentration of airborne particles is controlled, and which is constructed and used in a manner to minimise the introduction, generation, and retention of particles inside the room and in which other relevant parameters, e.g. temperature, humidity, and pressure, are controlled as necessary.
The first two thirds of the definition is, in essence, what a cleanroom is, It is a room that minimises the introduction, generation and retention of particles. This is achieved, firstly, by supplying it with exceptionally large quantities of air that has been filtered with high efficiency filters. This air is used to (1) dilute and remove the particles and bacteria dispersed from personnel and machinery within the room and, (2) to pressurise the room and ensure that no dirty air flows into the cleanroom. Secondly, a cleanroom is built with materials that do not generate particles and can be easily cleaned. Finally, cleanroom personnel use clothing that envelops them and minimise their dispersion of particles and micro-organisms. These and other similar measures that, minimise the introduction, generation and retention of contamination in a cleanroom are discussed here. Cleanrooms can also control the temperature, humidity, sound, lighting, and vibration. However, these parameters are not exclusive to cleanrooms, and are there-fore not discussed in any detail here.
………………………

Thanks cleanroomforum.com give me this opportunity to share these information, I will go ahead.