The sum of all the variable costs of production, such as materials, wages and transport.
Below is a diagram to show an example of a total variable costs curve for an individual firm. As can be seen this curve is linearly increasing at a proportionate rate because these costs are linearly increasing when more output is produced and therefore this line will indefinitely continue until the firm either has a reduction or increase in the amount of variable payments such as the amount each worker is paid or ho many workers they decide to hire.