Variable costs
Expenses that rise and fall with how much you use, buy or produce, such as groceries, fuel, hourly labour and the materials in each unit sold.
Variable costs move with activity. Drive more and fuel costs more; run the heating harder and the bill rises. For a business the clearest test is per unit: the materials in one more item, the processing fee on one more sale. The total grows with volume while the cost per unit usually holds roughly steady, which is the pairing that makes break-even arithmetic work.
In a household budget these are the costs that can be acted on this week, so they attract most of the attention. They are also the hardest to predict, which is why they are budgeted as an estimate and then compared against what actually happened. Splitting spending into fixed and variable is what makes a bad month diagnosable: if the total overran and the fixed side did not move, the answer is on the variable side.
The usual error is labelling every bill variable because the number changes. A flat-rate broadband bill that lands within a few dollars of the same figure every month is a fixed cost with noise on it. The test is not whether the amount varies, it is whether it varies because of something you did. A premium that rises on renewal changed on a schedule you do not control, so it belongs on the fixed side.