From Traderpedia

Definition: A term used for several futures contracts, but is most applicable to the small S&P 500 contract traded on CME.

The "e" part denotes that it is an electronically traded contract, and the "mini" part tells us that it is a relatively small contract, compared to the full-size S&P contract which is still pit-traded.

The contract trades in values of $50, which each 0.25 of a point being worth $12.50 to the trader.

Other 'mini' contracts are:

NQ - NASDAQ Composite Index Futures. This trades in half points, with each 0.5 being worth $10.00 to the trader.

YM - Dow Futures. This trades in full points, with each point being worth $5.00 to the trader.

ER2 - Russell 3000 Futures. This trades in one-tenth points, with each 0.10 being worth $10.00 to the trader.