Adverb
birationally (not comparable)
(mathematics) In birational terms.
Two varieties are said to be birationally equivalent if there exists a birational map between them.
Equivalently, they are birationally equivalent if their function fields are isomorphic. Source: Internet
This means that any algebraic variety can be replaced by (more precisely is birationally equivalent to) a similar variety which has no singularities. Source: Internet