Conjunction
In plain words
Two planets sitting in the same place. They act as one thing rather than two, which is why this rarely feels like something happening to you — it feels like your own idea.
In the tradition's terms
Two bodies within orb of the same degree: the two natures fuse and cannot easily be told apart in the effect.
In depth
Separation 0°. Traditionally neither benefic nor malefic in itself — it takes the character of the bodies involved, so Venus conjunct Jupiter and Mars conjunct Saturn are the same angle and opposite weather.