Put into play. Off its original square. Usually controlling the center to an extent.
Piece Development
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Think of the word developed. To develop something is to bring out possibilities of, to grow or expand, to bring into activity. Among other words, to activate.
ex: Moving a the king's knight from it's starting square to a more active square such as Nf3 when playing white or Nf6 when playing back is in turn "developing" a piece.
When one has all his pieces on active squares, it is said that one has developed all his pieces.
What does this mean? When is a piece considered developed?