Definition: Def. 10.10: Sixth Binomial
(Definition 10 from Book 10 of Euclid's “Elements”)
And if neither (term is commensurable), a sixth (binomial straight line).
Modern Formulation
The sixth binomial is a straight line whose length is \[\sqrt{\alpha}+\sqrt{\beta},\]
where \(\alpha,\beta\) denote positive rational numbers.
Mentioned in:
Proofs: 1 2 3 4 5 6 7 8
Propositions: 9 10 11
Thank you to the contributors under CC BYSA 4.0!
 Github:

 nonGithub:
 @Fitzpatrick
References
Adapted from (subject to copyright, with kind permission)
 Fitzpatrick, Richard: Euclid's "Elements of Geometry"
Adapted from CC BYSA 3.0 Sources:
 Prime.mover and others: "Pr∞fWiki", https://proofwiki.org/wiki/Main_Page, 2016