Piquant describes a flavor that is pleasantly sharp, tangy, or spicy. A piquant food is lively and stimulating to taste, but it does not necessarily have strong chili heat.
Mustard, vinegar, horseradish, pepper, and chili can make a dish piquant in different ways. The word describes the overall eating impression rather than a single ingredient or chemical process.
In recipes, a request for a piquant sauce usually suggests enough acidity, spice, or pungency to give contrast to milder or richer foods. Add strong ingredients gradually and taste for balance.