Mega-annum, usually abbreviated as Ma, is a unit of time equal to one million (106) years. It is commonly used in scientific disciplines such as geology, paleontology, and celestial mechanics to signify very long time periods in the past. For example, the dinosaur species Tyrannosaurus rex flourished approximately 65 Ma (65 million years) ago (ago may not always be mentioned; if the quantity is specified while not explicitly discussing a duration, one can assume that "ago" is implied; "mya" includes "ago" explicitly.). In astronomical applications, the year used is the Julian year of precisely 365.25 days.
Giga-annum, usually abbreviated as Ga, is a unit of time equal to one billion (109) years. It is commonly used in scientific disciplines such as cosmology to signify extremely long time periods in the past. For example, the formation of the Earth occurred approximately 4.55 Ga (4.55 billion years) ago. As is clear from the notation, billion as used here is on the short scale, also known as the American English preference.
Exa-annum, usually abbreviated as Ea, is a unit of time equal to (1018) years (one quintillion on the short scale, one trillion on the long scale). It is an extremely long unit of time. The half-life of tungsten is given as 20.9 Ea. Ea is also a very theoretical unit, since the age of the universe is unlikely to be more than 14.7 Ga, or 0.0000000147 Ea.