issue (def.)
Your direct descendants: your children, their children, and so on down the generations. Issue includes biological and adopted descendants.
Step-children and step-grandchildren are not issue, unless a Will expressly says otherwise.
The term matters where a Will passes a share to someone's issue. In an adeus Will this happens where a beneficiary who is your child dies before you. Their share passes down their branch of the family under per stirpes, including to children or grandchildren born after the Will was written. Those issue must survive you and reach the age of 18 to inherit. If none do, the share fails and is redistributed among your other residuary beneficiaries in proportion to their shares.