triangle
Two pairs of touching sides are the same length. The diagonals meet at right angles. Opposite sides are not parallel.
Draw a line touching one side to the opposite side of the square, which is not parallel to any side, and does not touch any vertex, so you will have two new quadrilaterals, each one has one pair of parallel sides and one pair of nonparallel sides.
two diagonal lines that aren't touching
Parallel lines are always the same distance apart and never touching. This is the definition of parallel in the plane. When we deal with spheres and other geometries, we must sometimes modify this definition. In any case, in the plane they do not meet.
The shape you are referring to is a kite.
triangle
Two pairs of touching sides are the same length. The diagonals meet at right angles. Opposite sides are not parallel.
Not touching anybody.
In Euclidean geometry, if and only if they are parallel.
If two lines go on forever without touching but are not parallel, they are skew, lines that do not lie in the same plane.
Draw a line touching one side to the opposite side of the square, which is not parallel to any side, and does not touch any vertex, so you will have two new quadrilaterals, each one has one pair of parallel sides and one pair of nonparallel sides.
two diagonal lines that aren't touching
Friction force. [opposite force]
1st position and parallel (or 6th position)
Friction force. [opposite force]
If you are referring to the segment passing through the center of the circle and touching the circumference of the circle then it's the diameter of the circle.There can be infinite diameters drawn inside a circle.