The sales volume or revenue at which total costs equal total revenue, so there is no profit or loss. Break-even units equal fixed costs divided by (price minus variable cost per unit).
Before launching a product, campaign, promotion or price change; when deciding whether a discount or a new channel can ever pay off.
$10,000 in campaign costs on a product with $50 margin needs 200 sales to break even; a 20% discount that pushes break-even volume up by half; an event that needs 150 tickets to cover the venue.
A simple sanity check every plan should pass. Flagged when promotions launch with no idea how many extra sales they need to pay for themselves.
