The term malus refers to a disincentive to those who wish to exchange a community currency back into a legal tender currency. A malus is typically a charge deducted from the amount due as part of the exchange.
Bonus is the opposite of malus.
Until December 31st, 2013 the Brixton Pound had a malus of 10% for those businesses who exchanged their Brixton Pounds into Pound Sterling. Although the exchange rate between Brixton Pound and Pound Sterling remains at 1 to 1, the malus functioned as a disincentive because business owner received less Sterlings per Brixton Pounds exchanged.