Notional
The notional is the full value of a position: its size times the price. A leveraged position is much bigger than the margin behind it, and the notional is what moves with the market, what fees are charged on and what your profit and loss is measured against.
Buy 0.25 BTC at 80,000 and the notional is 0.25 × 80,000 = 20,000 USDT, whether you put up 20,000 at 1x or 2,000 of margin at 10x. A 1% move changes the position’s value by 1% of the notional, 200 USDT, in both cases. Leverage is the notional divided by the margin.
Learn more: what is leverage? and maker vs taker fees.