Yachad means "together" and comes from echad, one. The most famous pair who were beyachad - as one - was Abraham and Isaac, "And the two of them walked together, vayelchu sh'nehem yachdav" (Genesis 22:8). Rashi explains yachdav as with an equal heart. This expression is used frequently in Israel as a title to lectures on seemingly contradictory concepts. For example, for years after Yitzchak Rabin's assassination, people asked "Democracy and Judaism, will the two walk yachdav?"
David in Psalm 133 declares, "How good and pleasant it is that brothers dwell also together, gam yachad." The word gam points to an extra level of unity. Rashi reads it as God being with the people when they are united among themselves. Others read it as a political aspiration for the Jewish people to be united in one polity.
Yachad also has spiritual connotations. Every morning, before reciting the Shema, we pray "Veyached - and make our heart one to love and fear Your Name". God declares, "I will give them one heart, lev echad" (Ezra 11:19). The blessing is to have a heart free of conflict and doubt. The Aramaic translation reads one heart as a "strong heart" - for a state of yachad strengthens both the individual and the nation.