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 2.5 million 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 is currently envisioned with a 2030s launch. It was earlier conceived aiming for deployment in the 2015-2025 time frame, but had a setback when NASA (an original participant) withdrew in 2011, at which time a scaled-down version called New Gravitational Wave Observatory was proposed in its place, but also was not accepted, and a full-scale LISA plan has been revived for further in the future.