at·ten·tion NOUN: 1. Concentration of the mental powers upon an object; a close or careful observing or listening. 2. The ability or power to concentrate mentally.
Some thesaurus entries: thorough, diligence, industriousness, interest
When I first encountered "attention" as a major field of cognitive science, my response was "what the hell is the big deal?" The more I study it / deal with it, the more important and interesting it gets.
Attention, in cognitive psychology, is thought about in many different ways: as the active allocation of general-purpose "cognitive resources"; as a passive filter of incoming stimuli; as the process of selecting what enters "awareness" or "consciousness"; as a "spotlight" that "illuminates" (in a metaphorical sense) a subset of sensory input.
The best antonym, I think, for "attention" is "distraction". At first glance, this might not make a lot of sense with the psychology-specific meanings of attention. But. What I think is that the psychological definitions grant (maybe unreasonably) a capacity for attentive cognitive control - the choice (by who? are there homunculus issues here) to direct attention towards one thing or another . . . and that they don't regard stability over time as important; if you're "distracted" it's not a reduction in attention, it's a re-direction of attention. Attention, in the non-specific definition is studied within psychology - but by clinicians, in the context of things like ADD & ADHD, not by cognitive psychologists (that I'm aware of.) I don't know, maybe I'm wrong; maybe it's just called "cognitive control", and not "attention".
This comes up, a lot, because I feel like I'm so easily distracted. Or rather, that I can focus effortlessly on some things, and not others. And that, unfortunately, my work too frequently falls into the "not others" category.
I didn't know anyone who used Ritalin as a "performance enhancer" in college, at Caltech, but I've heard that it's relatively common, some places. As much as I would never do it, I can kind of understand it; if I could achieve the same effects via another path - say, meditation - I definitely would.