Markup and margin are both ways to express the difference between cost and price, but they are different calculations with different denominators. Markup is the percentage added to cost to determine price. If a job costs $80 and you mark up 25%, the price is $100. Margin is the profit as a percentage of price. If the price is $100 and the cost is $80, the margin is 20%.
The confusion is costly. A 25% markup is not a 25% margin; it is a 20% margin. To target a 25% margin, you need a 33% markup. Contractors who use the words interchangeably often under-price. The simple rule: markup divided by (1 + markup) equals margin. Margin divided by (1 - margin) equals markup. Use whichever you prefer, but pick one and apply it consistently.