An "unless order" is an order which can be made by courts and tribunals, including an employment tribunal. It requires a claimant or respondent to take specified action (for example provide specified evidence) by a certain date failing which their claim or response will be struck out.

In other words an unless order is one which will become absolute unless the party to whom it is addressed shows the court or tribunal before a specified date why it should be set aside

See also notes at Procedure of Employment Tribunals/2004 rules/rule 13 - Compliance with orders and practice directions and at Procedure of Employment Tribunals/striking out