LISA (or Laser Interferometer Space Antenna) is a proposed European Space Agency space mission to detect gravitational waves by measuring the distance between three spacecraft using laser interferometry. Three probes, placed in a triangle, spaced 2.5M kilometers from each other, would continually measure the distances between them, awaiting gravitational waves to cause a change.
LISA Pathfinder was a 2015-2017 mission to test some of the necessary technology in space.
LISA had a setback when NASA (an original participant) withdrew in 2011, at which time, New Gravitational Wave Observatory, a scaled-down version was proposed. More recently, ESA has continued to look at LISA as a mission for the 2030s.