What is matter ?

It is not just solid, it is also liquid or gas. Here is alot of information on matter, some of it you may not need.
Anything which occupies space and has mass is known as matter. In physics, there is no broad consensus as to an exact definition of matter. Physicists generally do not use the saying when precision is needed, preferring instead to speak of the more clearly defined concepts of mass, energy, and particles.

Chemical matter is the part of the universe which is made of chemical atoms. This part of the universe does not include dark energy, dark matter, black holes or various forms of degenerate matter, such as compose white dwarf stars and neutron stars.

In particle physics and quantum chemistry, antimatter is matter that is composed of the antiparticles of those that constitute normal matter. If a particle and its antiparticle come into contact with each other, the two annihilate; that is, they may both be converted into other particles with equal energy in accordance with Einstein's equation E = mc2. Antimatter is not found naturally on Earth, except very briefly and in vanishingly small quantities (as the result of radioactive decay or cosmic rays). This is because antimatter which came to exist on Earth outside the confines of a suitable physics laboratory would almost instantly meet the ordinary matter that Earth is made of, and be annihilated.

In physics and cosmology, dark matter is matter that does not interact with the electromagnetic force, but whose presence can be inferred from gravitational effects on visible matter. According to present observations of structures larger than galaxies, dark matter accounts for the vast majority of mass in the observable universe.

In small quantities, matter can exhibit properties that are entirely different from those of bulk material and may not be well described by any phase.

In bulk, matter can exist in several different phases, according to pressure and temperature. These phases include the three familiar ones — solids, liquids, and gases. There are others, but most likely not applicable to your question. Phases are sometimes called states of matter, but this term can lead to confusion with thermodynamic states. For example, two gases maintained at different pressures are in different thermodynamic states, but the same "state of matter".

There you go. Susu