To discard refers to any manner by which a card is removed from play:

• a card is discarded as a burn card
• when a player folds his cards are discarded (or mucked)
• in draw poker when a player replaces cards, he first discards they take replacements
• if the dealer misdeals and exposes a card it is often discarded

Generally when cards are discarded they are left face down. However, in the case of a misdeal where a card is exposed, or when a player accidentally exposes his card to another, the card is announced to the table.