Organic growth is growth that comes from a company's existing businesses, as opposed to growth that comes from buying new businesses. It may be negative. Through Growth planning, businesses are able to achieve organic growth by selecting the best strategies available to them. For example, by examining Ansoff's matrix, businesses can select from market penetration, market development, product development and diversification to grow their revenue organically.
Organic growth does include growth over a period that results from investment in businesses the company owned at the beginning of the period. What it excludes is the boost to growth from acquisitions, and the decline from sales and closures of whole businesses.
When a company does not disclose organic growth numbers, it is usually possible to estimate them by estimating the numbers for acquisitions made in the period being looked at and in the previous year. It is useful to break down organic sales growth into that coming from market growth and that coming from gains in market share: this makes it easier to see how sustainable growth is.