Kappa: Sanskrit kalpa 'world-period', an inconceivably long space of time, an aeon. This again is subdivided into 4 sections: world-dissolution dissolving world, continuation of the chaos samvatta-tthāyī world-construction vivatta-kappa continuation of the formed world vivatta-tthāyī
How long a world-dissolution will continue, how long the chaos, how long the construction, how long the continuation of the formed world, of these things; o Bhikkhus, one hardly can say that it will be so many years, or so many centuries, or so many millennia, or so many hundred thousands of years A. IV, 156
A detailed description of the 4 world-periods is given in that stirring discourse on the all-embracing impermanence in A. VII, 62.
The beautiful simile in S. XV, 5 may be mentioned here: Suppose, o Bhikkhus, there was a huge rock of one solid mass, one mile long, one mile wide, one mile high, without split or flaw. And at the end of every hundred years a man should come and rub against it once with a silken cloth. Then that huge rock would wear off and disappear quicker than a world-period. But of such world-periods, o Bhikkhus, many have passed away, many hundreds, many thousands, many hundred thousands. And how is this possible? Inconceivable, o Bhikkhus, is this samsāra, not to be discovered is any first beginning of beings, who obstructed by ignorance and ensnared by craving, are hurrying and hastening through this round of rebirths.
Compare here Grimm's German fairy-tale of the little shepherdboy: 'In Farther Pommerania there is the diamond-mountain, one hour high, one hour wide, one hour deep. There every hundred years a little bird comes and whets its little beak on it. And when the whole mountain is ground off, then the first second of eternity has passed.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.