Acute Triangle: A triangle whose angles are all of a degree measure less than 90.

Altitude: a line that intersects one of the vertices of the triangle and perpendicular to its opposite side.

Circumcircle: a circle that contains the vertices of a polygon (in this case, a triangle ABC).

Orthocenter: the intersection of the three altitudes at a single point (in this case, point H).

Perpendicular: Two intersecting lines are perpendicular if they form a right angle (a 90 degree angle).

Polygon: a closed geometric figure whose sides intersect each other at their endpoints.

Vertex(vertices): points of intersection of two segments (sides) of a polygon at their endpoints.

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