Burn rate is the measure of net cash, a company lost in a period, usually a month.
Burn = Expenses - Cash inflow
Burn multiple is the ratio of revenue earned and the amount of money burned to earn that revenue.
The viral coefficient helps to quantify the extent to which a product reaches a maximum number of people by making minimum spending on marketing activities may be happening. Businesses often run referral campaigns to effect a high virality of the product and achieve a high virality coefficient.