What is called a basis? What is parallax? Define angular diameter.

(N/A) $(1)$ The distance between the two points of observation is called the basis.
$(2)$ When you hold a pencil in front of you against some specific point on the background (like a wall) and look at the pencil first through your left eye $(A)$ (closing the right eye) and then look at the pencil through your right eye $(B)$ (closing the left eye),you would notice that the position of the pencil seems to change with respect to the point on the wall. This shift in the apparent position of an object due to the change in the position of the observer is called parallax.
$(3)$ Angular diameter: The angle subtended by the diameter of a celestial body (like a planet or the moon) at a point on the surface of the Earth is called the angular diameter $(\alpha)$.