In mathematics, a Fermat quintic threefold is a special quintic threefold, in other words a degree 5, dimension 3 hypersurface in 4-dimensional complex projective space, given by the equation

This threefold, so named after Pierre de Fermat, is a Calabi–Yau manifold.

The Hodge diamond of a non-singular quintic 3-fold is

Herbert Clemens (1984) conjectured that the number of rational curves of a given degree on a generic quintic threefold is finite. The Fermat quintic threefold is not generic in this sense, and Alberto Albano and Sheldon Katz (1991) showed that its lines are contained in 50 1-dimensional families of the form

for ${\displaystyle \zeta ^{5}=1}$ and ${\displaystyle a^{5}+b^{5}+c^{5}=0}$. There are 375 lines in more than one family, of the form

for fifth roots of unity ${\displaystyle \zeta }$ and ${\displaystyle \eta }$.