Extensive stage, or by its full name, predominantly extensive stage of accumulation is pertains at one of the periodizations of capitalism, as proposed by Aglietta (1976). It is the first stage of capitalism and thus in it there is plenty of room for the extension of capitalist relations of production, meaning, of wage labour and therefore of commodity production. In other periodizations this is known as early or concurrential stage. It is characterized by high rates of growth (runaway, unfettered).

One of the best desciptions of this stage as it developed in England is in Marx's Capital I, Part VI: The so-called primitive accumulation

When the extensive stage becomes exhausted, it is followed by the intensive stage(of predominantly intensive accumulation).

References
Aglietta, Michel (1976) Régulation et crises du capitalisme Maspéro, Paris, A theory of capitalist regulation Verso, London, 1979
Marx, Karl (1867) Capital I, especially Part VI: The so-called primitive accumulation, Several editions.