Security of [energy] Supply is the ability of the power system to provide electricity to end-users with a specified level of continuity and quality in as a sustainable manner. 
Typically, one distinguishes between short- and long-term security of supply. The former one is the ability to cope with sudden disturbances like short circuits or lightning strikes. The latter one is the ability of the system to meet the demand of the end-consumers at all time, taking into account possible outages of generation plants or grid infrastructure.
Huoltovarmuus: Väestön toimeentulon, maan talouselämän ja maanpuolustuksen kannalta välttämättömien taloudellisten toimintojen ja niihin liittyvien teknisten järjestelmien turvaamista poikkeusolojen ja niihin verrattavissa olevien vakavien häiriöiden varalta. 
Security of supply: The capacity to maintain the basic activities that are indispensable for safeguarding the population’s living conditions, for sustaining the functioning of critical infrastructures, and the material preconditions for maintaining national preparedness and defence in case of emergency conditions and serious disturbances. 
Security of Supply is a nation’s ability to assure and be assured of contracted supplies of goods and services sufficient to discharge its defence and security commitments in accordance with its foreign and security policy requirements.