1. A line that will cut another line at 90deg. http://www.onlinemathlearning.com/im...p_image004.gif

2.angle bisector divided an angle into two equal parts http://www.mathwords.com/a/a_assets/...20bisector.gif

3.a median is a line drawn from the vertex of the triangle to the midpoint of the opposite side http://www.vias.org/comp_geometry/im...n_centroid.gif

note: there are three medians in a triangle, which all intersect at the same point

4.the altitude is a line going from a vertex of the triangle to the opposite side to the line is perpendicular to the side http://www.geogebra.org/en/wiki/imag...udePicture.jpg