Pecans are a tree nut in the hickory family. Pecans are large with reddish-brown shells and are excellent snacks as well as a popular ingredient in savory and sweet dishes. You can caramelize pecans and add them to a fresh green salad.
In addition to shelled and unshelled nuts, pecans are made into pecan meal, pecan flour and even pecan oil. Pecan meal and flour are used in baking and these products can be easily found online. Both Bob's Red Mill and King Arthur Flour produces pecan products.
If you don't have pecans you can substitute equal amounts of hickory nuts OR walnuts. Alternately, in baking, you can substitute 2 - 4 tablespoons of pecan oil per 1 cup shelled pecans needed. This will add the flavor but not the texture. You'll also need to reduce the fat used in the recipe by the same amount of pecan oil added.
Depending on the recipe, other optional nuts include:
1 pound shelled pecans = 4 cups 1 lb pecans in the shell = 2 1/2 cups shelled nuts 8 oz shelled pecans OR 2oz pecan pieces = 1/2 cup 1 cup chopped pecans = 109g 1 cup pecan halves = 3/4 cup chopped There are about 451-550 "large" pecan halves per pound
U.S. Pecans: Pecan sizing chart
National Pecan Company: Industry information and recipes.