A more thorough explanation:

Definition: A tax or duty, especially a customs duty.

Example: When a ship reaches the mainland, an impost is assessed on the goods it is carrying.

Explanation: An impost is a tax or duty that is imposed on goods that are being imported or exported. In the example given, when a ship reaches the mainland, the authorities assess an impost on the goods it is carrying. This means that the ship's owner has to pay a tax on the goods before they can be sold or distributed in the country. The purpose of an impost is to raise revenue for the government and to protect local industries from foreign competition.