Simple Definition:

Antimatter is a type of matter that has the opposite properties of normal matter. It consists of particles (tiny pieces) that have the same mass as regular matter but carry opposite electrical charges. When matter and antimatter come into contact, they annihilate each other, releasing a large amount of energy.

For example, a positron is a type of antimatter particle that has the same mass as an electron [a tiny particle that orbits the nucleus of an atom] but carries a positive charge (opposite to the negative charge of an electron). When a positron collides with an electron, they annihilate each other, producing energy in the form of gamma rays [a type of high-energy light].

Very Simple Definition:

Antimatter is a special kind of matter that acts differently from the things we usually see. When matter and antimatter meet, they disappear and create a big burst of energy.

Examples:
– When a special particle called a “positron” meets an electron, they both go away and make a bright light.
– If a special particle called an “antiproton” meets a proton, they both disappear and make energy.