The signature attack seen in the Dragon Ball and Dragon Ball Z franchises. We first see it performed by Master Roshi, to put out the fire on Mt. Paozu, unfortunately Roshi puts too much power in and blows the mountain away! The attack is charged with cupped hands behind the back, then brought to the front with the arms outstretched, and fired.
A Kamehameha is made from the condensed ki energy of it's creator, this becomes a Kamehameha wave when thrust outward, utilising it's inherent concussive force as a projectile weapon with intent to obliterate, crush, pierce, bruise, discourage, injure, or otherwise hinder it's creator's intended target, be it an enemy, vehicle, or mass of land.
The Kamehameha remains the signature move of Goku and his sons throughout the series. Multiple variations are seen over the series for example during his battle with Cell, Gohan uses a Super Kamehameha. During their battle with Broly, Gohan and Goten combine their Kamehameha waves. As a Super Saiyan 4 Goku uses Kamehameha X10, and when he fuses with Vegeta together they use the Big Bang Kamehameha. While the Kamehameha wave is usually done using two cupped hands, it has also been done using a single hand and on one occasion with two feet cupped together. In the 23rd World Martial Arts Tournament Goku uses the Kamehameha wave with his feet to boost him up in the air, leaving his hands free to fight. The Kamehameha is usually seen in the form of a beam but has on occasion been used in a ball form.
Be advised not to confuse a kamehameha in the fiction of all Dragon Ball media with one of six none-fictional past kings of Hawai'i, also named Kamehameha the first, second, third, fourth fifth and sixth.
On a related note: "Kamehameha Day" in Hawai'i is a day marked to celebrate the influence of these aforementioned kings, and not the concept of a Kamehameha in the Dragon Ball fiction.