A neutron star merger (or NS merger), the joining of two neutron stars, becomes likely when the orbit of a binary neutron star becomes sufficiently small that gravitational waves sap energy from the orbit, causing it to appreciably decay. When they merge, a gravitational wave event may result, and the ground gravitational-wave detectors, LIGO and Virgo can detect them within a certain radius. The sixth LIGO GW detection, GW170817, is ascribed to a neutron star merger, based upon the determined mass of the merging objects and the GW "sound" of the aftermath (ringdown). A slower inspiral than that of more massive black holes results in a substantially longer time within the detectable frequency range, and the ringdown is more complex due to the effects of the tidal forces on the material moving material around including ejecting some of it. The merger result can be a single neutron star, or a single black hole, possibly after a short (e.g., on the order of one second) life as a hypermassive neutron star.
Like black hole mergers, it isn't clear how the orbits become small enough for gravitational waves to pull them together, and the orbit decay is presumed to take a long time, e.g., more than 11 gigayears for GW170817.
Tidal deformability (TD, often symbolized by Λ, or tidal deformability parameter or just tidal parameter) is a calculated scalar value associated with the effects of tidal force on a neutron star, a function of its mass, radius, and Love numbers, and the binary tidal deformability (binary TD) is a combined scalar value depending upon the two, that affects details of the merger. Observation offers data that places constraints on the tidal deformability values, and in turn, on the values used to calculate them.