Nuclear material refers to the metals uranium, plutonium, and thorium, in any form, according to the IAEA. This is differentiated further into "source material", consisting of natural and depleted uranium, and "special fissionable material", consisting of enriched uranium (U-235), uranium-233, and plutonium-239. Uranium ore concentrates are considered to be a "source material", although these are not subject to safeguards under the Nuclear Non-Proliferation Treaty.
Different countries may use different terminology: in the United States of America, "nuclear material" most commonly refers to "special nuclear materials" (SNM), with the potential to be made into nuclear weapons as defined in the Atomic Energy Act of 1954. The "special nuclear materials" are also plutonium-239, uranium-233, and enriched uranium (U-235).
Note that the 1980 Convention on the Physical Protection of Nuclear Material definition of nuclear material does not include thorium.