Some firms merge.
Sometimes one firm acquires the other.
Sometimes this is referred to as a takeover.
Recently I heard that, back in 2002 when Andersens went into Deloitte, the words 'merger', 'acquisition' and 'takeover' were all banned. Instead staff and partners were instructed that the correct and only word to use was simply, the 'transaction'.
Perhaps this was to avoid references to my preferred description of so many such 'transactions' - a mergeover.