Union with Brahma, or company of Brahma? The following phrase comes from DN 13:


  “ayameva ujumaggo, ayamañjasāyano niyyāniko niyyāti takkarassa
  brahmasahabyatāya, yvāyaṃ akkhāto brāhmaṇena pokkharasātinā”ti.


T.W. Rhys Davids translated here as:


  ‘This is the straight path, this the direct way which makes for
  salvation, and leads him, who acts according to it, into a state of
  union with Brahmā. I mean that which has been announced by the Brahman
  Pokkharasādi.’


Bhikkhu Sujato translated here as:


  “This is the only straight path, the direct route that leads someone
  who practices it to the company of Brahmā; namely, that explained by
  the brahmin Pokkharasāti.”


What does "brahmasahabyatāya" mean? What does "sahabyatāya" mean?

This translation could be meaningful. "Union with Brahman" sounds like the modern Hindu concept of Moksha which is a union with God or Ultimate Reality. On the other hand, "company of Brahma" sounds like rebirth into the Brahma realm. What's the correct interpretation?
In DN 13 the term is originally used by Brahmins talking with each other, and so between them it meant whatever (non-Buddhist doctrine) they each understood it to mean.

At the end of the sutta the Buddha teaches them the four brahmaviharas and defines or explains that these are the path to "companionship with Brahma" -- i.e. I think he thus redefines brahmasahabyatāya (whatever it used to mean) in Buddhist terms, in a way that can be understood and agreed by the Brahmins.