There are various translations of the word on the Internet: “tranquility”, “one-pointedness”, “unification”. Mostly the term is used in the definitions of jhanas so it is quite important to understand the meaning of it properly. If we search for all occurrences in suttas we find that only in two cases the word is used not in jhana’s definition context, exactly in Digha Nikaya:
Hearing that, the gods of the Thirty-Three agreed in unison
Idaṁ sutvā devā tāvatiṁsā ekaggā samāpajjiṁsu (DN 18, DN 19)
All other occurrences are in the jhana’s definitions:
Unification of mind with these seven factors as prerequisites is called noble right immersion with its vital conditions and also with its prerequisites.
yā kho, bhikkhave, imehi sattahaṅgehi cittassa ekaggatā parikkhatā—ayaṁ vuccati, bhikkhave, ariyo sammāsamādhi saupaniso itipi, saparikkhāro itipi (SN 45.28, MN 117)
“Unification of the mind is immersion.
cittassa ekaggatā ayaṁ samādhi (MN 44)
and their mind is immersed in samādhi.
samāhitaṁ cittaṁ ekaggaṁ (AN 4.12, SN 35.134, AN 3.130, MN 28, MN 19, AN 8.11, MN 4)
gain immersion, gain unification of mind
labhati samādhiṁ, labhati cittassa ekaggataṁ (SN 51.13, SN 48.11, SN 48.50, SN 48.9, SN 48.10)
With mind unified and serene
ekaggaṁ susamāhitaṁ (SN 8.4, AN 3.59, AN 3.58)
unification of mind
cittassa ekaggatan’”ti (MN 125)
Thus Digha Nikaya confirms by using of the word in a different context that the primary idea of the word is getting together, be united, unified and in application to mind to unite the mind