Tenma Defense is an Energy Shield used by Gohan.
Gohan charges his ki and lets forth a large white barrier to either knock away opponents or deflect attacks.
The first instance of Gohan using this technique was in Dragon Ball Z: Broly - Second Coming when he battled the maniacal Legendary Super Saiyan Broly. When the villain fired an Eraser Cannon at Gohan, he tried to deflect it into the air but it followed him back down again. Trying to stop the attack, Gohan used his Tenma Defense to stop the attack and defend his friends.
Appearances in Video Games
Tenma Defense was named in Battle of Z, where it appears as one of Adult Gohan's special attacks. When the attack is used near a teammate, the teammate is healed as well as himself.
"Create a barrier! Additional input will make the barrier bigger. Hold the button to maintain it while using up Ki."
In Dragon Ball Xenoverse 2, it appears under the name Energy Dome and appears as an Evasive Skill used by Adult Gohan in his Uniform Skillset and by Future Gohan. In Xenoverse 2, it can block ki blast moves and pressing the button again will allow the user to expand the barrier to knock back opponents, while holding it will allow the user to maintain it using ki.
"Creates a barrier that can block Ki Blasts. Pressing the button again expands the barrier. This can be used to knock back opponents in addition to stopping projectiles."
— Dragon Ball Xenoverse 2 Tutorial description
As part of the Masters Pack DLC, the Future Warrior can learn this skill by completing School Lesson 2: "Lesson 2" of Future Gohan's Training. Interestingly, it is referred to as Future Barrier in the plot description of Lesson 2, despite the skill itself appearing under the name Energy Dome in the Lesson's Basic Rewards section.