In a trading account what is the difference between margin and collateral?

The word margin has several uses:

• Buying on margin refers to money borrowed.

• Initial margin requirement refers to the amount of collateral required to buy on margin (cash and/or marginable securities). It's 50% unless you're trading leveraged securities or your broker imposes stricter margin requirements.

• Margin maintenance requirement (MMR) refers to the amount of collateral you must have to maintain a margin position (25% for long positions and 30% for short positions unless you're trading leveraged securities or your broker imposes stricter margin requirements). A margin call occurs if your account equity drops below your MMR.

Here's an article which explains long and short margin calculations.