What is defined as the willingness of firms to provide goods and services at various prices?

The concept defined as the willingness of firms to provide goods and services at various prices is known as supply. Supply reflects how much of a particular good or service producers are willing to sell at different price levels. As the price increases, typically, the supply also increases because firms are more motivated to produce and sell more to maximize their profits.

Understanding supply is crucial in economics because it helps explain the behavior of producers in the market and how they respond to changes in price. The relationship between supply and price is often depicted through the supply curve, which generally slopes upwards from left to right.

The other concepts listed in the question serve different purposes: demand refers to consumers' willingness to purchase goods and services at different prices; equilibrium describes the point where supply and demand balance each other; and market trend refers to the general direction in which a market or economic variable is moving over time. Each of these concepts is important, but supply specifically focuses on the producer's side of the market.