Increasing the ratio of fissile Uranium (U-235) in natural uranium is known as :

Raising the share of fissile uranium-235 in natural uranium is called enrichment.

Enhancement is not the technical term for increasing the fissile isotope ratio.

Concentration refers generally to increasing amount per volume, not isotope separation.

Accumulation means gathering or building up, not raising the fissile fraction.

Enrichment separates the lighter uranium-235 from the heavier uranium-238 to boost its proportion.

Reactor fuel needs a few percent enrichment while weapons need over ninety percent.