Single Shot in the context of espresso brewing refers to a single cup of Espresso extracted from approximately 7-8 grams of coffee powder. A typical espresso Single-shot can hold about 25 -35ml of coffee, and it is brewed using a single basket.
These days Single Shot is scarce. Usually, those espressos in a coffee shop you call a double shot. This is especially true in regions like the United States and the United Kingdom, where a double shot of Espresso has become the norm. In older coffee cultures such as in continental Europe, Single-shot is the standard serving of coffee, and Espresso is usually prepared in one go using a Single filter basket.
The traditional single Shot is the basis of all Espresso-based drinks such as cappuccino, Americano, caffè latte, caffè mocha, and espresso macchiato. In recent years, as the demand for higher and larger caffeinated beverages has increased, the Double Shot has primarily replaced the single Shot as the standard amount of Espresso in milk and water espresso drinks.