Sense of wonder is a fannish science fiction term used to describe the feeling that some SF evokes in the reader. In current fandom it is often spelled "sensawunda".
Sense of wonder is the feeling you had when you read your first SF novel as a teenager. You imagined the starships, the alien races, the exotic planets, and you wondered and dreamed. You'd never read something like this before, and you were excited, and you wanted more. If it was the real thing, you are probably still reading SF.
As SF readers become older, it becomes harder and harder to find this effect. See also the cynical line stating that "The Golden Age of Science Fiction is 12".
"The Encyclopedia of Science Fiction" (ed. Clute/Nichols) points out that the term is interesting in two ways: it describes SF by its effect rather than its content, and it can be found in a number of books that are usually regarded as badly written.