supplement; addition; extra amount
Refers to something added to make up a shortage or to supplement an existing amount. Often used in financial or quantitative contexts.
Since the budget was insufficient, I applied for a supplement.
The ingredients were a little short, so I bought some extra.
Compound of 足し (tashi, 'adding', from verb 足す) and 前 (mae, 'front' or 'portion'). The exact historical derivation is uncertain, but it likely refers to an amount added in front of or on top of the existing quantity.