what does position stand for?

Seems to be simple but I wonder what the precise definition of “Transform.position” is?

I have a character, say 5.7 inch high on a terrain of 0 inch in height, then the position of him is (x, 0, z) or (x, 5.7/2, z)?

The “position” of an object is its location relative to some point of reference, generally it’s the position of the model’s pivot point. That point of reference can be the world origin, or in other cases the position relative to some other parent object. None of that has anything to do with the “height” (I assume you meant physical height, not “height” as in position like altitude) of the object.

With your character, if I assume that the pivot point is at the bottom of the character’s feet then the position is (x, 0, z), if you assume that the pivot is half way between the mesh’s max/min-y extents, then the position is (x, (5.7/2), z).

cool, thanks, how about the x, z, same as y?

Well, no, you’ve said nothing about where on your terrain your character is so those will be some numbers that I have no insight into.

Well, to make sense, I have a terrain sized 500m * 500m, height 0, pure flat. Also have a character sized 0.4m (depth, front side to back side) * 0.6m (width, left arm to right arm) * 1.8m (height), and his pivot point is his feet. His relative position to the terrain is (0, 0, 0), then what is his position in Unity?

(0, 0, 0)? or (0.2, 0, 0.3)? or other?

Your terrain has some position relative to the world, you haven’t stated what this is so I’ll assume it’s at (0, 0, 0). If your character’s position relative to the terrain is (0, 0, 0) then the character’s world position is (0, 0, 0).

Thanks again.

Terrain position is defaulted to (0, 0, 0) in the world position. So you assumed right.