The phrase "illegal detention" refers to the act of unlawfully holding or imprisoning someone against their will. It is when a person is kept confined without proper legal authority or valid reasons. Full definition
The Federal Government on Wednesday said it was still studying the verdict of the ECOWAS court which on Tuesday ruled against it over what it called illegal detention of former National Security Adviser, Col Sambo Dasuki. (nigeriapoliticsonline.com)
The billionaire kidnapper is asking the court to award him N300m as exemplary damages for his alleged illegal detention and the alleged harm caused by his media trial. (nigeriapoliticsonline.com)
You could argue the detention itself is a form of harm, and that the refugees should be allowed to enter the USA proper, but courts have generally held that you can compensate people for illegal detention with money. (politics.stackexchange.com)