The rate at which a new company uses up its venture capital to finance overhead before generating positive cash flow from operations. In other words, it's a measure of negative cash flow.

Burn rate is usually quoted in terms of cash spent per month. For example, a burn rate of 1 million would mean the company is spending 1 million per month. When the burn rate begins to exceed forecasts, or revenue fails to meet expectations, the usual recourse is to reduce the burn rate (which, in most companies, means reducing staff).