A 'default' is what happens when the person being sued fails to respond to the lawsuit within the required time, allowing the other side to win automatically.
A default is a procedural failure to appear, answer, or otherwise defend within the time set by court rules after being properly served with a summons and complaint. It appears when the court clerk or judge enters a 'default' on the record, which can later lead to a 'default judgment' granting the opposing party what they asked for without a trial.
If a default is entered against you, you lose the chance to present your side of the story, and the court may issue a binding judgment against you without hearing your evidence. Acting immediately upon learning of a lawsuit is critical to avoid default, as courts generally require a prompt motion to set it aside, often with a valid excuse.
Many people think a default means they have lost the case permanently or that it proves they are at fault. In reality, a default is just a procedural step that can sometimes be undone if you act quickly and show good cause, but it is not a final determination of liability.
Get the actual rule
This page explains the concept. When it matters to your case, read the rule that governs it in your court — we hold Texas, federal and Washington court rules word for word, and every state's official resources are on the state pages. If someone cites a case at you, check that it exists first.
Related
General explanation, not legal advice, and not specific to any state — procedures and deadlines vary by jurisdiction and court. If you can get a lawyer or free legal aid, do: every real option.