In California nurseries, the term "Mexican lime" is often used interchangeably with "Key lime." Both refer to the same citrus fruit, scientifically known as Citrus aurantifolia. The terms "Mexican lime" and "Key lime" are used to describe the same variety of lime with small, round, and highly acidic fruit.
As for the best lime variety to use for tacos, it depends on personal preference. Key limes (Mexican limes) are commonly used in Mexican cuisine, including tacos, due to their distinctive flavor and acidity. However, other lime varieties such as Persian lime (Citrus × latifolia) are also popular and widely available. Persian limes are larger and less acidic than Key limes, offering a slightly different flavor profile. Ultimately, you can choose the lime variety that suits your taste preferences and availability.