Torture is defined in Art. 1 of the UN Convention against Torture as:
"... any act by which severe pain or suffering, whether physical or mental, is intentionally inflicted on a person for such purposes as obtaining from him or a third person information or a confession, punishing him for an act he or a third person has committed or is suspected of having committed, or intimidating or coercing him or a third person, or for any reason based on discrimination of any kind, when such pain or suffering is inflicted by or at the instigation of or with the consent or acquiescence of a public official or other person acting in an official capacity."
Therefore, for something to be classified as torture under international law, it must fulfill at least the four criteria of causing severe pain or suffering, being intentionally inflicted (and not just by coincidence or neglect), be inflicted for one of the purposes listed, or something substantially similar to these, and finally, be conducted, instigated or acquiesced to by someone acting in official capacity.
The severity of the pain or suffering undergone by a victim is often difficult to determine either purely objectively or subjectively, but needs to take account of both factors. Often major indicators for whether or not treatment amounts to torture will depend, for example, on the age, physical and mental health of the victim, as well as the intensity and duration of the ill-treatment inflicted.
The requirement of intentionality is important for an act of torture, especially because it is central the criminal liability of the individual perpetrator. This does not however mean that persons cannot suffer the same severity of pain or torment from unintentional conduct, such as accidental neglect, but only that this may then be termed inhuman treatment, rather than torture.
The listed purposes for which the severe pain or suffering must be inflicted for something to constitute torture are not exhaustive, but give an indication of the kind of purposes for which torture is often used. The essence of the nature of theses purposes is usually that they are connected to some kind of official interest, such as the solving of crimes, or the oppression of opposition.
Out of the four criteria, especially the last one, that an act of ill-treatment is only considered to be torture when inflicted by a public official, is especially important as the prohibition of torture, besides holding the state accountable for the human right violation, also requires the criminal prosecution of the individual committing the torture.
It is important to note that the prohibition of torture usually also encompasses the prohibition of inhuman and degrading treatment, which do not require all of the criteria above, but may nonetheless cause the victim just as much pain or suffering as outright torture.