Initial Margin

What Does Initial Margin Mean?
The percentage of the purchase price of securities (that can be purchased on margin) that the investor must pay for with his or her own cash or marginable securities.

Also called the "initital margin requirement".

Investopedia explains Initial Margin
According to Regulation T of the Federal Reserve Board, the initial margin is currently 50%. This level is only a minimum and some brokerages require you to deposit more than 50%.

For futures contracts, initial margin requirements are set by the exchange.