Is a reduction in all the components of blood. So there are decreased red blood cells, white blood cells, and platelets (anaemia, neutropenia and thrombocytopenia).
Red blood cells: Hb <12g/dL for women and <13.5g/dL for men
White blood cells (neutrophils): ANA <1500
There are many causes of pancytopenia, including leukaemia, aplastic anaemia, SLE, familial haemophagocytic syndrome, myelodysplastic syndrome, pernicious anaemia, among several others (consult your local wiki for further information).