Lemon extract by John Spottiswood.

lemon zest (1 teaspoon extract = 2 teaspoons zest) OR oil of lemon (One part flavoring oil is roughly equivalent to four parts extract, but this may vary according to the products used. To be safe, begin by substituting 1/8 teaspoon of oil per teaspoon of extract, then add more drops of oil until you're satisfied with the flavor.) OR orange extract OR vanilla extract OR lemon-flavored liqueur (Substitute 1 or 2 tablespoons liqueur for each teaspoon of extract)