A broadstruck error occurs when a coin is struck without the collar to form the rim and edge that is part of the shape of the coin. Coins can be broadstruck on either Type One or Type Two planchets. When a coin is broadstruck the blank being fed into the collar will spread and distort outward as it is being struck because the collar isn't in the correct position to retain it.

Images courtesy of Byers Numismatic Corp

Significant examples:
Rare Date Barber Half 1897-S Broadstruck (illustrated above).

Sources and/or recommended reading: