Definitions
1.VerbTo order (goods or services) in advance, before they are available.
2.VerbTo sort or arrange beforehand.
3.NounAn order for goods or services placed in advance.
4.NounA binary relation that is reflexive and transitive."The relation of logical implication over sentences is an example of a preorder."
5.AdjectiveSuch that, recursively, the root is visited before the left and right subtrees.