If you needed to describe matter in one word, that word would be substance. Matter is anything that has mass and takes up space. As long as there is no vacuum there is matter present. It exists in three main forms which are solid, liquid and gas with unique characteristics in each form. Matter in one form can be transformed to another form by changing its temperature or pressure. Many people do not consider gases to be matter but they actually are. If you have a deflated ball and inflate it with air, the space inside the ball will be occupied. This means that air will have met one of the qualities of matter. When you weigh the ball before and after inflating you will realize that it weighs more when inflated thus air will have proven that it possesses the second quality of matter which is to have mass.
Matter is made up of atoms which are the smallest form in which matter can exist in. An atom is not visible to the naked eye even in solid state. Matter exists either as atoms of pure elements or as a combination of elements. From the definition of matter you may wonder if there are things that exist that do not have matter. The answer to that question is yes, there are things that exist that are not matter. Take light for example, it has no mass and though it seems to occupy space it actually does not. Sound is also something that actually exists yet it has no mass and does not occupy space. Emotion is another example of something that is not matter though it is sometimes described using terms that are used with reference to matter. A heavy heart implies weight though we know that emotions do not posses this quality.
Matter has both primary and secondary qualities. The primary qualities of matter are those that can actually be measured. These qualities are mass and volume and their components such as height and width. The secondary qualities of matter however cannot be measured. These qualities are identifiable and distinct though you cannot quantify them mathematically. Qualities such as smell, taste and color fall in this category.
When in solid form, matter has a definite shape, volume and is hard to compress, in the liquid state matter will also have a definite volume, be difficult to compress but will be having no definite shape. Gases will be easy to compress, have a definite volume and similar to liquids they will take up the shape of the container holding them as they have no definite shape of their own. The reason for this is that in gaseous and liquid state the atoms of any matter are loosely held together.
Matter also has another meaning when used in regular speech. When someone says they have a matter to discuss, matter here will mean an issue. This is usually used when there maybe a need to withhold details while hinting to something relatively serious.