因果关系是一个事件（即“因”）和第二个事件（即“果”）之间的关系，其中后一事件被认为是前一事件的结果。

一般来说，因果还可以指一系列因素（因）和一个现象（果）之间的关系。对某个结果产生影响的任何事件都是该结果的一个因素。直接因素是直接影响结果的因素，也即无需任何介入因素（介入因素有时又称中介因素）。从这个角度来讲，因果之间的关系也可以称为因果关联（causal nexus）。

原因和结果通常和变化或事件有关，还包括客体、过程、性质、变量、事实、状况；概括因果关系争议很多。对因果关系的哲学研究历史悠久，佛教和西方哲学家如亚里士多德在2000多年前就已经提出了因果，该问题仍是现代哲学的重要课题。

Causality (also referred to as 'causation', or 'cause and effect') is the agency or efficacy that connects one process (the cause) with another (the effect), where the first is understood to be partly responsible for the second, and the second is dependent on the first. In general, a process has many causes, which are said to be causal factors for it, and all lie in its past. An effect can in turn be a cause of many other effects, which all lie in its future.