what is a bird in science terms

Answer 1

Both the terms Aves and Neornithes apply to birds.
Hope this helps.

Answer 2

Another possibility (can't tell exactly what the question is asking) is that a bird, in science terms but not everday terms, is a reptile.

The newer classifications attempt to be monophyletic, so the term reptile includes all reptiles and all animals descended from reptiles, which would include birds.