An eclipse occurs when one body obscures light from another body. The best known phenomenon of this type is a Solar eclipse, which occurs when the Moon passes between the Earth and the Sun, blocking the Sun's light from a region on the planet's surface.
Eclipse in Mass Effect
Little is known about the larger Eclipse network. Eclipse cells operate on various planets under the direction of a local officer, who supervises their operations. In addition to mercenary work, they are also active smugglers of weapons, drugs, and fugitives. A typical Eclipse cell is a commando unit that emphasizes the use of mobility, biotic powers, and combat engineering (tech powers).
Eclipse is a significant force in the poorly governed Terminus Systems, but their forces are far too limited to threaten a major planetary government, let alone an interstellar government.