Any theory which says that the object of perception plays a causal role in the perception itself.
The object may cause us to have a certain experience without itself being perceived (we may have to infer its existence, or 'construct' it from experiences rather as we 'construct' the average man from real men: also see: phenomenalism).
Or we may perceive the object but our experience in doing so only counts as perceiving it if it is itself caused by the object. Or it may simply be that whenever we do perceive an object it has a causal role to play in our doing so (without that role forming part of the analysis of perception).
Objections to the first two views include that of ensuring that the causal chain is of the right kind.
A J Ayer and L J Cohen, 'The Causal Theory of Perception' (symposium), Proceedings of the Aristotelian Society, supplementary volume (1977)