The term "intellectual" refers to a specific way of relating to the world, particularly to those fields and hobbies which interest one. The term does not specifically refer to level of
intelligence, and while a person needs a better-than-average
intelligence to be an intellectual, many highly intelligent people are not intellectuals -- and many highly intelligent people
are.
The opposite of an intellectual may be someone who lacks creativity (a mundane) or may be someone who relates to the world with pragmatism. The opposite of a "pragmatist" may be someone who lacks any common sense (a naif) or may be an intellectual.
An intellectual is someone with a better-than-average creative intelligence who loves to relate to the world at large and to his/her hobbies predominantly through abstract reasoning, analytic logic, associative logic, the Scientific Perspective, imagination, and a sense of awe and wonder. Intellectuals love theory for itself rather than demanding concrete applications.
Intellectuals are often called geeks both by mundanes and by themselves. Intellectuals tend to love complex studies in literature and/or science and/or philosophy. The most common intellectual recreational interests include computers, SF, fantasy, computer games, RPGs, comic books, anime, and sub-titled foreign films. The interests of intellectuals, idealists, and artists often overlap.
In short, an intellectual is someone who actually cares about meaning, philosophy, symbolism, abstract theory, and other things which may lack any immediate practical or concrete applications. In contrast, a pragmatist considers such things worthless because they don't produce discernible material or concrete results, and a mundane considers such things worthless because they seem "odd" and take too much effort to understand with no obvious financial results.