Strait:
A narrow channel of sea which separates two land masses is known as a strait. This natural geographical feature is formed by the movement of tectonic plates or erosion caused by water or glaciers.

Formation:
1. Tectonic Activity: Some straits are formed due to the movement of tectonic plates. When the Earth's crust shifts, it can create narrow channels of water between land masses. Examples include the Strait of Gibraltar, which separates Europe from Africa, and the Bering Strait, which separates Asia from North America.

2. Erosion: Erosion caused by water or glaciers can also create straits. Over time, the force of flowing water or the movement of glaciers can carve out narrow channels between land masses. The erosion can be caused by rivers, waves, or even the movement of ice. The English Channel, which separates the United Kingdom from Europe, is an example of a strait formed by erosion.

Importance:
1. Trade and Transportation: Straits are important for international trade and transportation. They provide a navigable route for ships, allowing them to pass between different bodies of water. Many major shipping routes pass through straits, making them crucial for global commerce.

2. Defense and Security: Straits also have strategic importance in terms of defense and security. They can serve as natural barriers, protecting one land mass from potential threats from the other side. Controlling a strait can give a country a significant advantage in terms of defense and military operations.

3. Biodiversity and Ecology: Straits often serve as important habitats for marine life. They can support diverse ecosystems and serve as breeding grounds for various species. Many straits have unique environmental conditions, such as strong currents or nutrient-rich waters, which attract a wide range of marine organisms.

Examples:
1. Strait of Hormuz: This strait is located between Iran and Oman and connects the Persian Gulf with the Gulf of Oman. It is one of the most strategically important chokepoints in the world, as it is a major route for the transportation of oil and gas.

2. Strait of Malacca: Situated between Malaysia and Indonesia, the Strait of Malacca connects the Indian Ocean with the South China Sea. It is one of the busiest shipping lanes in the world and is vital for international trade, particularly for countries in East Asia.

In conclusion, a strait is a narrow channel of sea that separates two land masses. It can be formed by tectonic activity or erosion and has significant importance in terms of trade, defense, and ecology. Examples like the Strait of Hormuz and the Strait of Malacca highlight the geopolitical and economic significance of these natural features.