Battery capacity is an electrical charge (quantity of electricity) that battery can deliver under specified conditions. Usually battery capacity is expressed in ampere-hours (A*h or Ah). Capacity equals to discharge current (in amperes) multiplied by discharge time (in hours).
Battery producers rate nominal battery capacity for some standard conditions. Usually (but not necessary), these conditions are: 20-hour discharge time, 20°C discharge temperature and 1.75 V final battery voltage (voltage limit at which discharge is considered to be complete).