Preheat the oven to 325°F. In a medium bowl, combine sweetened condensed milk, lemon juice, lemon zest or extract, and egg yolks; stir until mixture thickens. Pour into cooled graham cracker crust or cooled pie crust.
In a medium bowl, beat egg whites and cream of tartar at high speed until soft peaks form. Gradually stir in sugar, beating on medium speed, one tablespoon at a time; beat 4 minutes longer or until sugar is dissolved and firm, shiny peaks form.
Spread meringue over pie, sealing carefully at the edge of crust to prevent shrinkage. Bake for 12 to 15 minutes or until meringue is lightly browned. Let cool. Keep leftovers covered in the refrigerator.
