Gravity and anti gravitation
Gravitation is nothing but a force commonly known as "gravitational force". A body attracts another body if they are having a potential difference; potential difference is also a form of energy. If both the potential energy is same then they are likely to meet at a mean position of their gravitational force locus.
As far as the term "anti-gravitation" is concerned, let us consider 2 charged particles both having the same magnitude of potential energy. Consider them to be both positively charged. In this case the will "reflect" each other or in simple words the will go away from each other which you may consider it as "anti-gravitation".
I don't think so that there is any term "anti-gravitation". But if it exist also, it may be defined
I think that perhaps dark energy is a form of anti gravitation. It is still a hypothetical concept, but it is postulated to exist throughout the space and is the reason why the universe keeps expanding and the rate of expansion is still increasing.
Outer space is still a stranger to us. You may not know what makes the universe expanding. It’s just hypothesis or in other word "math", math can make even the unimaginable true.
Consider other forces like electric force or magnetic force ( both are now considered to be the same force united by Einstein's theory of relativity) Both these forces are attractive n repulsive in nature. It depends on the polarity. These forces are symmetrical in nature.
Similarly the gravitational force may also be symmetrical. It may also depend on the polarity. We are living in the world of positive mass. Our universe may also contain negative mass( probably dark matter which is believed to be opposite of matter). So due to this difference in polarity the gravitational force may also be repulsive or anti gravity when matter interacts with the dark matter. This ' anti gravity ' is believed to be the reason for the increased rate of expansion of the universe though there is no solid proof in this regard.
Another way of thinking about anti gravity is imagining it as negative curvature of space time. Gravity is positive curvature.
The electromagnetic force is the one that opposes gravity. If we are able to stand on the earth its only because of the electromagnetic force of repulsion between the electrons in the outermost shell of our shoes and the electrons in the outermost shell of the ground.
Remember that the electromagnetic force is almost 10^38 times stronger than gravity. If that would not have been the case then we would have sucked in to the core of the earth.