Conversion on an e-commerce site would be purchase of the product that the website offers. Conversions are usually defined as either creating a direct revenue (macro conversion) or potential for revenue down the line (a micro conversion).
In terms of CRO, it is always referred to the macro conversion, as the primary objective of CRO is to increase the revenue. Micro conversions are not usually solved for in optimizing process, as it they do not bring direct revenue and are thus not measurable in terms of ROI. It is accepted fact that the efforts to drive the macro conversions up will result in driving micro conversions up, as they are usually a complimentary or subsidiary to the macro conversion.