What do you mean by spin or nuclear isomers of hydrogen ?

Explanation

Hydrogen has two nuclear spin isomers called ortho and para hydrogens. Ortho hydrogen : When the spins of the nuclei are in the same direction (parallel spins), the dihydrogen molecule is known as ortho hydrogen.

Para hydrogen : When the spins of the nuclei are in opposite direction (antiparallel spins), the hydrogen molecule is known as para hydrogen. Due to the different directions of nuclear spins in the ortho and para hydrogens, they are known as spin isomers or nuclear spin isomers.