A dependent is someone you support. Typically, a dependent is a child or other relative.
Rules to determine whether you can count someone as a dependent are below. Generally, you have a dependent if you've provided at least half of a person's total needs for the year (food, shelter, clothing, etc.).
Here's how to know if you should count someone as a dependent on your tax withholding:
Must be a US Citizen, US national, or a resident alien
Must have a tax identification number (social security number, individual taxpayer identification number, or adoption taxpayer identification number)
Can file their own tax returns, and even be married, but must not have filed a joint return unless it was only to claim a refund
To claim a child as a dependent:
Must have lived with you for half the year
Must be related to you (son, daughter, stepchild, foster child, brother, sister, stepbrother, stepsister, or a descendant of those)
Must be 18 or under at the end of the year (or under 24 if a student who was full-time for at least 5 months of the year)
Must be younger than you (or your spouse if married filing jointly), unless the child is disabled
For other relatives or an unrelated person:
Must not have income over $4,050
Must not be a qualifying dependent of another person (you can't claim the person if someone else could)
Must be related to you or have lived with you all year as a member of your household