Have you ever heard the word Terrorism? If yes, Can you define it? The term Terrorism is one of such terms which do not universally accepted definition. It was also differentiated on basis of political opinion like Mujahideens who fought against soviets were called terrorist by USA. But this can be contradicted as can a revolutionary people in India also called as terrorist.
Many other organizations and countries had made definition of it with their own perspective and ideas.
By United Nations:
Before UN, League of Nations announced a draft convention in 1937 for the prevention and punishment of terrorism. According to observations of Javier Ruperez a stakeholders of UN was unable to agree a definition on paper but UNSC has passed the resolutions of 1267 , 1368 and 1371 and 19 legal instruments in order to prevent terrorist acts.
By India:
The NSGA in 1986 defines a ‘Terrorist’ as any person who with intent to overawe the Government as by law established or to strike terror in the people or any section of the people, does any act or thing by using bomb, dynamite or other explosive substance or inflammable substances or firearms or other lethal weapons or poisons or noxious gases or other substances (whether biological or otherwise) of a hazardous nature.

India also made the UAPA 1967 act, TADA 1987 act and in the case of Hitendra Vishnu Thakur vs State Of Maharashtra, Supreme Court of India opined terrorism as abnormal phenomenon which also affects the psychology of victims.
Special Tribunal for Lebanon (STL) also observed terrorism as international crime with three key elements: perpetration of criminal act, intent to spread fear among the population, when act involves a transnational element.
By European Union: on the background of 9/11 attack
As seriously intimidating a population, or unduly compelling a Government or an international organization to perform or abstain from performing any act, or seriously destabilizing or destroying fundamental political, constitutional, economic or social structures of a country or an international organization.

FBI has classified terrorism as International (inspired by, or associated with, designated foreign terrorist organizations or nations) and Domestic (as further ideological goals stemming from domestic influence).
The famous Terrorism Act (2000) of UK as the use of threat of action involving violence against a person , endangering others life, creating serious risk to health etc.
Academically.
One of the most important attempts of them is the survey conducted by Alex Schmid and Albert Jongman (1988).
Challenges in Defining Terrorism
Terrorism as a Virtue : Defining terrorism based on certain political or ideological framework as founding principle could thus be a problem in itself as creating a political consensus regarding this issue would be hardly possible.
Approaches :
Objective like Attempt to threaten the state specific Action and Action specific like plane hijack and bombing.
Conclusion
While the failure to establish a commonly accepted definition is evident; terrorism has indeed emerged as one of the serious challenges and an objective definition of terrorism is not only possible it is also indispensable to any serious attempt to combat terrorism
