Definition
The documentation calls them derivatives products without expiration date, which rely on funding payments to ensure convergence to the underlying spot price over time. One contract is one unit of the underlying spot asset. You never take delivery: the contract settles in cash, and a position can be held for an hour or for a year. Long gains when the price rises, short when it falls, and every contract open is one of each.
Because nothing expires, funding is what holds the contract near the asset. On Hyperliquid it is paid every hour, between the two sides, and the venue collects nothing on the payments.
You post margin rather than the contract's value. The margin required to open a position is position_size * mark_price / leverage, with the leverage an integer between 1 and the market's maximum, which varies by asset from 3x to 40x. The maintenance margin, the floor the position may not fall through, is half the initial margin at that maximum, so between 1.25% of the notional on a 40x market and 16.7% on a 3x one. Hyperliquid's perpetuals are linear, with USDC as collateral.
Three prices matter and they are not the same. The book price is where trades happen. The spot oracle price, published by the validators about every 3 seconds, is what funding is computed on. The mark price is what margin, liquidations, order triggers and unrealized profit and loss are measured against, and the documentation warns it can differ significantly from the book price in volatile times or on a highly leveraged position. A perpetual is a leveraged contract: everything put behind a position can be lost, and in a backstop liquidation the maintenance margin is not returned either.
Example
An example, in round figures. A long of 0.1 BTC at $100,000 is $10,000 of notional. At 10x the initial margin is $1,000, and the maintenance margin on a 40x market, 1.25% of the notional, is $125. A move of 5% in the price is $500, which is half of the margin put up, in either direction. The market does not know how much of your own money stands behind the $10,000: leverage changes only that.