Formal, semi-formal, black tie, white tie -- all of these are called dress "codes" for a reason: They follow strict guidelines. Smart-casual, on the other hand, is a little more undefined. There are no hard-and-fast rules for smart-casual dress; it's a bit more of a value judgement that typically pops up in business situations where the formal factor is dialed down a notch, but not quite dialed down to business casual levels.
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Think of smart-casual as a hybrid -- it typically means that your wardrobe is publicly presentable on a business level, but takes a few casual cues (think sport coat rather than business suit). For both men and women, the good news is that it often serves as an opportunity to get a bit more expressive with your work-friendly wardrobe.
If smart-casual has been presented to you in the business environment or even just for an upcoming event, don't assume -- the term sometimes means different things to different people, so always ask around beforehand.