In genealogy and wills, a person's issue are all their lineal descendants.
Issue typically means a person's lineal descendants—all genetic descendants of a person, regardless of degree. Issue is a narrower category than heirs, which includes spouses, and collaterals (siblings, cousins, aunts, and uncles). This meaning of issue arises most often in wills and trusts. A person who has no living lineal descendants is said to have died without issue.
A child or children are first-generation descendants and are a subset of issue.
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