clandestine
adj.done in secret, often because it is illegal or not allowed. You use this to describe things like hidden meetings or private plans.
adj.kept secret or done secretively, especially because illicit. Typically describes activities, meetings, or relationships rather than physical objects.
“The two spies had a clandestine meeting in the park.”
“The group held clandestine meetings in a basement to plan their protest without the police finding out.”
“Historians recently uncovered documents detailing the clandestine operations carried out by the resistance during the final months of the occupation.”
Often used attributively before a noun; frequently collocates with 'meeting', 'operation', or 'affair'.
“In Abigail Weinstock’s sparkling one-woman show, 30-year-old Eliana has been caught making clandestine recordings in her neighbour’s bedroom and cannot deny it.”
“So began America’s formal entry into peacetime espionage, including clandestine intelligence missions.”
From Latin clandestīnus (“secret, concealed”); compare French clandestin.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.