Expense Ratio

What Does Expense Ratio Mean?
A measure of what it costs an investment company to operate a mutual fund. An expense ratio is determined through an annual calculation, where a fund's operating expenses are divided by the average dollar value of its assets under management. Operating expenses are taken out of a fund's assets and lower the return to a fund's investors.

Also known as "management expense ratio" (MER).

Investopedia explains Expense Ratio
Depending on the type of fund, operating expenses vary widely. The largest component of operating expenses is the fee paid to a fund's investment manager/advisor. Other costs include recordkeeping, custodial services, taxes, legal expenses, and accounting and auditing fees. Some funds have a marketing cost referred to as a 12b-1 fee, which would also be included in operating expenses. A fund's trading activity, the buying and selling of portfolio securities, is not included in the calculation of the expense ratio. 

Costs associated with mutual funds but not included in operating expenses are loads and redemption fees, which, if they apply, are paid directly by fund investors.