What is the difference between a condition (shart), a pillar (rukn), and a duty (wajib)?

A condition (shart) is that whose absence necessitates the absence of the conditioned thing (mashroot), but its existence does not necessitate anything by itself, and it is external to the essence (mahiyyah). A pillar (rukn), on the other hand, is an integral part of the essence. Scholars sometimes use these terms interchangeably by way of metaphor, because the existence of the essence depends on both. An obligatory act (wajib) is an act of the legally responsible person (mukallaf) which Allah Almighty has commanded him to perform as an obligation; it is a legal ruling (hukm taklifi). Some jurists differentiate between a pillar and an obligatory act by stating that a pillar is never waived under any circumstances, whereas an obligatory act can be waived by forgetfulness. Others consider them to be synonymous or differentiate between them in certain chapters of jurisprudence but not in others.