Act is a law passed by the parliament whereas ordinance is issued by the president or governor.
Act requirements
Act is a law passed by the parliament by all parliamentary houses i.e. upper-house and lower-house, whatever applicable e.g. house of lords, house of representatives, house of commons, National Assembly, Senate etc. This depends on the constitution of the country that which house(s) required to pass the law.
Ordinance requirements
Ordinance is issued by the president on the advice of the Prime Minister (in most countries where parliamentary system is adopted) or by the governor of the province. Usually ordinance has at time-limit of effectiveness to be endorsed by the concerned parliament or chief executive of the country/province (prime minister or chief minister) may advice the president/governor to undo the ordinance.
Executive Order
Executive order is usually ordered by the president of the country presidential system applied, under the constitution and should not violate a law.
What is better? Act or Ordinance
Enforcement of both, the act and the ordinance, is same. Act is more reliable and democratic way in sophisticated manner as parliamentarians debate on the law and anomalies can be rectified before it have been passed which is not so with the Ordinance.
Most ordinance are criticized as proved to be biased to bypass the parliament usually due to the opposition reaction.
Why Ordinance is issued instead of parliamentary Act
Ordinances are also issued when the govt. is not performing well and either the country is in crisis or the govt. politically. In parliamentary system, the government tend to bypass parliament through ordinance as the govt. may fail to pass and exposed to lost majority.
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