Triangles

Acute

All acute angles

Right

One right angle and two acute angles

Obtuse

One obtuse angle and two acute angles

Equilateral

All angles are 60 degrees

Scalene

No equal sides

Isosceles

Two equal sides; the angles of those two sides are equal

Equilateral

All equal sides

Median

vertex of a triangle to the midpoint of the opposite side. intersection of all three is the centroid.

Altitude

Segment from vertex of a triangle to the other side of a triangle. It must form a right angle with the other side of the triangle. The intersection of all altitudes is called the orthocenter.

Perpendicular Bisector

Segment from one side of a triangle that cuts the said side in half and forms a right angle with it. intersection of all three is the circumcenter.

Midsegment

midsegment connecting two sides of a triangle is parallel to the third side and is half as long. The midsegment cuts the two sides of the triangle into equal parts

Heron's formula (know sides, need area)