Make or become more diverse or varied. Diversification is the process of spreading the total investment money available across different asset classes, countries, industries, and individual companies. Diversification also entails choosing investments that are, as far as possible, negatively correlated, which means that when investment A is performing poorly, investment B is likely to be performing well. 
Diversify "to make diverse," Economic sense is from 1939. 
Diversification c.1600, noun of action from diversify. Economic sense is from 1939.