Tanden, in Chinese pronounced as Dan Tien – shared for those who love Google – is the centre of the body. It is maybe better to verbalise it as the centre of gravity which lies inside the body.
Look at the image >> Tanden is the white dot*. So not just about an inch below the navel, but also inside the body. Not on the surface. It goes straight through by Seichusen and so lies on centre line, in the centre.
Drawing a line straight down to the ground from the top of the head till the ground, the centre of gravity is slightly ahead of the middle of the ankle.
The black dot is the location where the centre of gravity is on the floor. When it comes to stances and weight distribution, that concept is a more complete approach than say "60/40" because it talks about forward, back, left AND right (instead of only forward and back).
*Note we call the white-dot-location "Seika Tanden", but also "Ge-Tanden". Usually, most simply call it "Tanden".
Ge-Tanden is the 'lowest' Tanden as some instructors teach three levels:
Think about it.
Is it a muscle? Bone? Tendon?
Is it ANY part of the body known by doctors, physical therapists or science?
No. Tanden is imaginary... and put plainly: it doesn't exist.
BUT THERE'S GOOD NEWS: It helps you FOCUS.
When you imagine Tanden to be your source of power, you're forced to move from the center which then allows you to move more smoothly and use your body weight more effectively.
Let's look at 2 core Karate techniques: kick and punch.
When you punch, your attention goes to the upper body and fist. Yes, you may hurt your opponent a lot, but what about the next movement? If you focus on punching "from Tanden" you're more likely to create correct stability and power output that STILL allows you to move and stay centered.
When you kick it's the same thing...
Conclusion: Although Tanden is a concept and not a muscle that you can control, it helps you to focus on the AREA that needs attention. When you punch 'from' and 'with' Tanden, you'll get that Karate punch result. The same is true for Seichusen, which is an imaginary line that does not actually exist, but helps you to control your body movement better.
The concept of Tanden raises your awareness.