its a designation for a type of futures contract
the "e" means they are only traded electronically
and the mini - means that there is another similar contract with a higher value
for example - the emini sp - is only traded electronically and has a value of $50 per point - whereas the big sp has a value of $250,000 and can be traded electronically when the SP pit is closed, but not electronically when the SP pit is open
there are eminis in lots of contracts - from gold, bonds etc - but in general the term is used for the emini sp and emini nasdaq - the smaller contract of the dow could also suit this label - but as most professional to date dont trade the dow - you can be pretty sure that when used it is applying to the sp and nasdaq