AD is a line segment which is perpendicular to the side BC of ABC .This line segment AD is called as altitude of 

Altitude is also called as height of a triangle. In the given figure AD is the height of ∆ ABC  from vertex A to side BC.

•    Orthocentre is the meeting point of all three altitudes of a triangle.
•    In the given figure H is the orthocentre of ∆ ABC.
•    In obtuse-angled triangle the orthocentre lies outside the triangle.
•    In Acute-angled triangle the orthocentre lies inside the triangle.
•    In a Right-angled triangle, the right angle lies at the vertex at which the right angle is formed.