In geometry, a cevian is a line segment which joins a vertex of a triangle to a point on the opposite side of the triangle. Medians, symmedians, angle bisectors, altitudes are all special cases of cevians. The name cevian comes from the Italian mathematician Giovanni Ceva, who proved a theorem about cevians which also bears his name.

The length of a cevian can be determined by Stewart's theorem: in the diagram, the cevian length d is given by the formula

Less commonly, this is also represented (with some rearrangement) by the following mnemonic:

If the cevian happens to be a median (thus bisecting a side), its length can be determined from the formula

or

since

Hence in this case

If the cevian happens to be an angle bisector, its length obeys the formulas