Answer
$${\text{ circle}}$$
Work Step by Step
$$\eqalign{
& {x^2} = 25 - {y^2} \cr
& {\text{Add }}{y^2}{\text{ to both sides of equation}} \cr
& {x^2} + {y^2} = 25 \cr
& or \cr
& {x^2} + {y^2} = {\left( 5 \right)^2} \cr
& {\text{The equation is written in the form }}{\left( {x - h} \right)^2} + {\left( {y - k} \right)^2} = {r^2} \cr
& {\text{Therefore,}} \cr
& {\text{The graph of the equation }}{x^2} = 25 - {y^2}{\text{ is a circle}} \cr} $$
