Given a circle and a fixed point $P$  not lying on it. Find the geometrical locus of the orthocenters of the triangles $ABP$, where $AB$ is the diameter of the circle.
1. Consider a circle with center \( O \) and a fixed point \( P \) not lying on the circle. Let \( AB \) be a diameter of the circle.
2. Let \( PA \) and \( PB \) intersect the circle again at points \( X \) and \( Y \) respectively.
3. We need to find the orthocenter of triangle \( ABP \). Recall that the orthocenter of a triangle is the point where the altitudes intersect.
4. In triangle \( ABP \), the altitude from \( A \) to \( BP \) and the altitude from \( B \) to \( AP \) intersect at the orthocenter.
5. The line \( AX \) is perpendicular to \( BP \) and the line \( BY \) is perpendicular to \( AP \). Therefore, the orthocenter \( H \) of triangle \( ABP \) is the intersection of \( AX \) and \( BY \).
6. By the properties of polar lines in projective geometry, the intersection of \( AX \) and \( BY \) lies on the polar of point \( P \) with respect to the circle.
7. The polar of \( P \) with respect to the circle is a line that is defined as the locus of points from which tangents drawn to the circle are equal in length.

Thus, the geometrical locus of the orthocenters of the triangles \( ABP \), where \( AB \) is the diameter of the circle, is the polar of point \( P \) with respect to the circle.

The final answer is the polar of point \( \boxed{ P } \) with respect to the circle.