The term margin is used in business, finance, and investing in at least three different ways:
1. Margin in business
2. Margin in financial accounting
3. Margin in investing

Here is the answer for margin in investing in stock market.
In stock market, margin refers to buying shares of stock or other securities with a combination of the investor’s own funds and borrowed funds. If the stock price changes between its purchase and sale, the result for the investor is leverage, meaning that the investor’s percentage gain or loss is magnified compared to the percentage gain or loss had the investor purchased shares without borrowing.