Lime juice is the juice of any one of a number of cultivars of the lime, a greenish fruit with a sour pulp and acidic juice. There are many different types of lime, with the most common being:

Citrus latifolia, also known as the Persian lime, the Tahiti lime, and the Bearss lime (after John Bearss, who developed it in 1895). These are the most common supermarket limes, and are seedless.

Citrus aurantifolia, also known as the Key lime or Bartender's lime. These are smaller and more yellow than the Persian lime, with many seeds and are not only highly acidic, but also interestingly bitter at the same time. 

Citrus hystrix, also known as the Kaffir lime, or Makrut lime. These are about as small as the Key Lime, but have a warty skin and are intensely sour. The kaffir lime (both zest and especially leaves) are commonly used in Malaysian and Thai cooking, but the juice is more commonly used as a cleanser. 

All lime juice is sour, having a pH of between 1.8 and 2, which makes it more sour than [ingredient=lemon-juice lemon juice]. For cocktail usage, the lime's strong green, grassy aspect works well with herbal flavors (think [ingredient=gin gin] and Green [ingredient=chartreuse Chartreuse]). It is not as neutral of an acidifier as lemon juice. 

Some popular cocktails containing Lime juice

  • Christmas Caipirinhas — Cachaça, Ginger beer, Ice cubes, Lime juice, Sugar, Lime, Mint, Ginger
  • Under the Peruvian Sun — Solbeso, Bianco Vermouth, Licor 43, Aztec Chocolate bitters, Passion fruit liqueur, Lime juice
  • Aul Man — Irish whiskey, Bitter Truth EXR, Lime juice, Ginger-Honey Syrup, Lime peel, Candied ginger
  • Pisse Tafia — Martinique Rum, Herbal liqueur, Bitters, Cider, Lime juice, Cane syrup
  • Florodora (Dead Rabbit Version) — Gin, Framboise, Bitters, Ginger ale, Ginger syrup, Lime juice, Nutmeg
  • Kashmiri Caipirinha — Cachaça, Simple syrup, Lime juice, Tamarind, Turmeric, Chili pepper
  • Przewalski's Mule — Bison grass vodka, Becherovka, Kummel, Ginger beer, Lime juice, Lime
  • Well-Hung Trellis — Dimmi, Genever, Lime juice, Simple syrup
  • Chef's Kiss — Fernet Branca, Herbal liqueur, Demerara syrup, Lime juice, Lime
  • Caribbean Coffee Crush — Añejo rum, Falernum, Fernet Branca, Simple syrup, Lime juice, Pineapple