1. Definitely or positively
   "the results are surely encouraging";
   - certainly, sure, for sure, for certain, sure enough, sure as shooting [informal]
    
2. In truth (often tends to intensify)
   "they said the car would break down and surely it did";
   - indeed, so
    
3. Without a doubt
   "the grammar schools were surely not intended for the gentry alone";
   - assuredly
    
4. Without fail
   "slowly but surely"

See also: beyond a doubt, beyond a shadow of a doubt, beyond doubt, indisputably, indubitably, without a doubt

Encyclopedia: Surely