Accumulation is in the context of corporate finance, refers to profits that are added to the capital base of the company rather than paid out as dividends.
In the context of investments, accumulation refers to the purchase by an institutional broker of a large number of shares over a period of time in order to avoid pushing the price of that share up.
In the context of mutual funds, accumulation refers to the regular investing of a fixed amount while reinvesting dividends and capital gains.