The monthly cash loss of a company that isn't profitable yet.
Burn rate is how fast you're spending money faster than you're making it. Typically shown as a monthly dollar figure: 'we're burning $300k/month.' Paired with your cash-on-hand, this tells you your runway, how long until you run out. Every unprofitable startup obsesses over burn rate and runway.
Burn is the countdown. Most founders under-estimate it until they can see the wall. Know your burn to the dollar, your runway to the month. Revisit monthly. When burn creeps up faster than revenue, you have a problem worth solving this quarter, not next.