Vertex: The vertex is the corner of the triangle. Triangle has three vertices.

Base: Generally, the bottom side of a triangle is called base.

Altitude: Altitude is a line through a vertex and perpendicular to the opposite side (or base) of the vertex.
Intersection of three altitudes is called the orthocenter of the triangle.

Median: A median is a line from the vertex to the midpoint of the opposite side.
Intersection of three medians is called the centroid of the triangle.

Orthocenter: Intersection of three altitudes is called the orthocenter of the triangle.

As shown in figure, point O is orthocenter of triangle.

Centroid: Intersection of three medians is called the centroid of the triangle.

Incircle: The biggest circle which touches all the sides of a triangle is called incircle of a triangle.

The intersection of three angle bisector is center of incircle. It is also called incenter of triangle.

Circumcircle: A circle which passes through all the vertices of the triangle is called circumcircle.

The intersection of three perpendicular bisectors is called circumcenter.

Interior Angles:

• A triangle has three interior angles.
• Sum of three interior angles of a triangle is 180°.
• Shortest side is always opposite to the smallest interior angle.
• Longest side is always opposite to the biggest interior angle.

Exterior Angles:

• An exterior (or external) angle is the angle between one side of a triangle and the extension of an adjacent side.
• Sum of all the exterior angle is 360°
• An exterior angle of a triangle is equal to the sum of the opposite interior angles.