Which ingredient is NOT included in a Regular Martini?

In a Regular Martini, the traditional ingredients are gin and dry vermouth, with gin being the primary spirit. Vodka can also be used as an alternative to gin, particularly in vodka martinis, but it does not define a Regular Martini. Dry vermouth plays a crucial role in providing the characteristic flavor of the drink.

Olive juice is often associated with a Dirty Martini, which is a variation that incorporates olive brine, adding a salty and savory aspect. However, in the context of a Regular Martini, the inclusion of olive juice is not standard. A Lemon Twist is frequently used as a garnish for a Regular Martini, contributing a fresh citrus aroma and flavor profile. Therefore, the correct answer identifies that olive juice is not a traditional ingredient in a Regular Martini.