What is the focus of a parabola?

the focus of a parabola is a point inside the parabola and the distance from the parabola's point from focus and the line named directrix is the same. Infact The parabola is the set of points of the plane equidistant from a point called the focus and a line called the directrix.

Look this image

if the parabola's canonic equation is ##y=ax^2+bx+c## to find the coordinate of Focus you can to use this formula:

##x=-b/(2a)## and ##y=(1-(b^2-4ac))/(4a##