What is Excess Reserves?

Legal Definition
In banking, excess reserves are bank reserves in excess of a reserve requirement set by a central bank.

In the United States, bank reserves for a commercial bank are held in part as a credit balance in an account for the commercial bank at the applicable Federal Reserve bank (FRB). This credit balance is not separated into separate "minimum reserves" and "excess reserves" accounts. The total amount of FRB credits held in all FRB accounts for all commercial banks, together with all currency and vault cash, form the M0 monetary base. Holding excess reserves has an opportunity cost if higher risk-adjusted interest can be earned by putting the funds elsewhere. For banks in the U.S. Federal Reserve System, this earning process is accomplished by a given bank by making short-term (usually overnight) loans on the federal funds market to another bank that may be short of its reserve requirements. Other banks may instead choose, however, to hold their excess reserves to facilitate upcoming transactions or to meet contractual clearing balance requirements.
-- Wikipedia
Legal Definition
Banks having more cash reserve than the law requires. Low economic activity, weak loan demand, high interest rates, or tight monetary policies are typical contributing causes for this condition.