Subjective idealism is a theory in the philosophy of perception. It describes a relationship between human experience of the external world, and that world itself, in which objects are nothing more than collections (or bundles) of sense data in those who perceive them.
A famous proponent of subjective idealism was 18th century Irish philosopher George Berkeley.
This theory has much in common with phenomenalism, the view that physical objects, properties, events, etc. (whatever is physical) are reducible to mental objects, properties, events, etc. Thus reality is ultimately made up of only mental objects, properties, events, etc.