Set inclusion is a concept in mathematics that describes the relationship between two sets. If all elements of one set, say A, are also elements of another set, B, we say that A is a subset of B. This is denoted as A ⊆ B. For example, if A contains the numbers 1, 2 and B contains 1, 2, 3, then A is included in B. In contrast, if A contains elements that are not in B, then A is not a subset of B. Additionally, if A and B are identical, they are considered equal sets, denoted as A = B. Understanding set inclusion is fundamental in various fields, including mathematics, computer science, and logic.