Decentralized exchange (DEX) is an exchange market that does not rely on a third-party service to hold the customer's funds. Instead, trades occur directly between users (peer to peer) through an automated process (https://www.cryptocompare.com/exchanges/guides/what-is-a-decentralized-exchange/).
Trade order is an order to exchange to move funds between users' accounts.
Loan order is an order to the lending platform to take the lender's funds and provide the borrower with that funds directly or indirectly.
Orderbook is the place, where users can publish their orders.
Relayer is a person or entity, which hosts off-chain orderbook.
Long position is an exchange deal, which has the intent to get some funds in exchange for another, and then to own new funds.
Short position is an exchange deal, which has the intent to get some funds in exchange for another, but to own them just for a short period, until price changes and funds would be sold with the profit.
Collateral is an asset that a borrower offers as a way for a lender to secure the loan.
Margin trading is usage of the borrowed funds when trading in a short position with the goal to increase profits while also increasing risks.
Margin call is a situation, when trader, who uses margin trading loses his own funds (collateral) in a short position, and market forces close of trader’s short position.
Swap provider is an external to the bZx platform DEX that can be used for trade orders execution.
Liquidity source is a funds reserve, that gives ability to execute orders (event unexpected).
Liquidity provider is a connection to source of liquidity, that provides ability to execute deals on side of liquidity source.
Price feed provider on-chain source of exchange rates.
WETH wrapped ETH. This is ERC20 token, which represents ETH. It's needed for trading ETH in the same way as any other ERC20 token. More at https://weth.io/.