Rūpa (रूप) means "form". As it relates to any kind of basic object, it has more specific meanings in the context of Indic religions.
According to the Monier-Williams Dictionary (2006), rūpa is defined as:
any outward appearance or phenomenon or colour (often pl.), form, shape, figure RV. &c &c ...
to assume a form ; often ifc. = " having the form or appearance or colour of ", " formed or composed of ", " consisting of ", " like to " ....
In Hinduism, many compound words are made using rūpa to describe subtle and spiritual realities such as the svarupa, meaning the form of the self. It may be used to express matter or material phenomena, especially that linked to the power of vision in samkhya, In the Bhagavad Gita, the Vishvarupa form, an esoteric conception of the Absolute is described.
Overall, rūpa is the Buddhist concept of material form, including both the body and external matter.
More specifically, in the Pali Canon, rūpa is contextualized in three significant frameworks:
rūpa-khandha – "material forms," one of the five aggregates (khandha) by which all phenomena can be categorized (see Fig. 1).
rūpa-āyatana – "visible objects," the external sense objects of the eye, one of the six external sense bases (āyatana) by which the world is known (see Fig. 2).
nāma-rūpa – "name and form" or "mind and body," which in the causal chain of dependent origination (paticca-samuppāda) arises from consciousness and leads to the arising of the sense bases.
In addition, more generally, rūpa is used to describe a statue, in which it is sometimes called Buddharupa.
In Buddhism, Rūpa is one of Skandha, it perceived by colors and images.
According to the Yogacara school, rūpa is not matter as in the metaphysical substance of materialism. Instead it means both materiality and sensibility—signifying, for example, a tactile object both insofar as that object is made of matter and that the object can be tactically sensed. In fact rūpa is more essentially defined by its amenability to being sensed than its being matter: just like everything else it is defined in terms of its function; what it does, not what it is.