Stability is best defined as the opposite of instability, which is the occurrence of large structural deformations which are not the result of material failure.
The degree to which a material or perfume is effected over time by heat, light and air. Or if the perfume is in a product(application) how it is effected over time by interaction with the base of that product. Instability may show itself by change in colour, odour, separation of the product or viscosity.
From the early times instability of the search results in general, and of the number that represents results in particular has been a subject of special concern. Certainly the Web is a highly dynamic system, growing at an incredible pace, but also the crawlers change their specifications and schedule unexpectedly. A world crawling round can last from 15 to 45 days and in this meantime.