The term for the difference between the bid and ask price is the spread.
More precisely, it is called the bid–ask spread: the gap between the highest price a buyer currently offers and the lowest price a seller will accept. For example, a quote of $49.95 bid and $50.05 ask has a spread of $0.10.
The spread is a practical cost of immediacy. A trader who wants to sell immediately generally receives the bid, while one who wants to buy immediately pays the ask. Market makers or other liquidity providers may earn the difference for standing ready to trade, while competition and heavy trading usually narrow it.
A common mix-up is calling the spread “liquidity.” Liquidity describes how easily an asset can be traded with little price impact; a narrower spread is often evidence of better liquidity, but the two terms are not synonyms. “Margin” concerns borrowed funds or collateral, and “volatility” measures price fluctuations.