In sets it means intersects. like what is common in both sets
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The Upside down T symbol indicates perpendicular lines.
It is the symbol for 'perpendicular'.
That's a u
The only "mathematical" (it's really more geometrical) symbol I can think of fitting the description means "perpendicular."
The 'upside down' triangle symbol is the (greek?) letter Nabla. Nabla means the gradient. The gradient is the vector field whoose components are the partial derivatives of a function F given by (df/dx, df/dy).