You may be surprised to learn that peanuts, cashews, almonds, pecans, walnuts and Brazil nuts are not actually nuts.
While those are all commonly referred to as nuts in the culinary world, none of them is really a true nut in a biological sense.
What defines a nut biologically?
Nuts, botanically speaking, are made up of both the seed itself and a hard outer layer that is the fruit, which does not split open when ripe.
Some examples of true nuts are acorns, hazelnuts and chestnuts.
Walnuts, Pecans and Almonds
Walnuts, pecans and almonds, in contrast, are edible seeds from a bigger fruit. Most of the inedible fruit is removed at harvest so when you buy walnuts and almonds in the shell, you're buying the seeds enclosed in woody endocarp, which is the remaining layer of the fruit wall.
Nuts or Seeds?
Brazil nuts are also seeds. They come from a capsule like fruit, in which several of the triangular seeds grow inside, arranged like the segments of an orange.
Peanuts are the seeds of a legume, or member of the bean family. Pistachios, cashews, macadamias and pine nuts are all seeds rather than nuts.
Nuts or Seeds... They Are Healthy
One truth about both "true nuts" and the seeds we call nuts is that they are both rich in healthful omega 3 fatty acids as well as vitamins and protein.