This paper presents an overview of an optical instrument for disaster and security monitoring from the geostationary orbit. The described instrument is the payload of the GEOBS (GEOstationary OBServatory) concept, which is the result of an ESA exploratory activity conducted at the ESA-ESTEC Concurrent Design Facility (CDF) in early 2020 in order to assess the technical and programmatic feasibility of optical mission concepts from GEO. Recent improvements in the maturity of critical technologies (e.g. adaptive optics, large CMOS detector arrays, ultra-stable materials,…), together with the development of adequate manufacturing facilities and the required manufacturing techniques in Europe and Japan, have provided the ground for further investigation of the current technical and programmatic feasibility of a geostationary observation system for disaster and security monitoring. The instrument on-board the GEOBS satellite provides medium spatial resolution imagery (10-20 m Ground Sampling Distance at equator) in three visible narrow bands (Red, Green and Blue - RGB), one Panchromatic band and one band in the Near Infrared (NIR). The instrument acquires images using a step-and-stare observation technique, enabling persistent monitoring of selected areas. The instrument is sized to provide a footprint >100 km at Equator and observe the area from 30° to 55° North in Latitude and from 10° West to 36° East in Longitude, with observations possible from 4:00 a.m. to 8 p.m. local solar time. The instrument architecture is based on a Korsch telescope, with an active optics correction loop (wavefront sensor, deformable mirror, 5 degrees-of-freedom mechanism on the secondary mirror), and a focal plane based on multiple CMOS detectors, assembled in either 1D or 2D large arrays to cover the full field of view. The instrument concept also includes ultra-stable structures and light-weight sun-shields to reduce straylight and thermos-elastic effect.