Definition of 'Investor Shares'
Mutual fund shares purchased by individual investors as opposed to institutional shareholders. In general, the amount invested in the fund is what distinguishes investors from institutional shareholders. In many cases, a fund will have a minimum investment that must be made to qualify as an institutional shareholder.
Investopedia explains 'Investor Shares'
For example, Vanguard has a shareholder category called Admiral Shares, which requires a minimal investment level of $100,000 in a fund. At this level, a fund shareholder will benefit from a slightly lower expense ratio as well as certain service benefits.
If a fund carries a load, it will apply to investor shares, whereas institutional shareholders will generally have this fee reduced or waived completely because of the size of their investment.