general resemblance; largely the same
Often used in the set phrase 大同小異 (largely the same with minor differences). As a standalone noun, it is somewhat literary and less common in everyday speech.
See also: 大同小異
Their opinions are much the same with only minor differences.
Since they are largely the same, please don't worry about the small details.
uniting with a common goal
Primarily appears in the compound 大同団結 (a grand coalition or union for a common purpose). Rarely used independently in modern Japanese.
See also: 大同団結
Call for a grand coalition.
大同 is sometimes used to express uniting for a common purpose.
Daidō era (806-810)
Historical Japanese era name. Used only in historical contexts.
The first year of the Daidō era corresponds to 806 CE.