Used as a noun or の-adjective for things that are the same, such as dates, places, opinions, or items of the same kind. 同じ is the more general equivalent.
The baby started crying at the same time the phone rang.
Expressions
一緒にするdo together; do something jointly · put together; lump together; mix; join · regard in the same light; put in the same category; confuse · arrange for (two people) to marry; bring together as a couple
一緒になるcome together; join; merge; meet up · get married; become a married couple
味噌も糞も一緒lumping good and bad together; treating everything as the same
糞も味噌も一緒lumping good and bad together; not distinguishing between good and bad; mixing the worthy with the worthless
盆と正月が一緒に来たようas if two great things happened at once; as if Christmas and one's birthday came together