The planet that rules the sign that a planet is currently in is called its planetary dispositor. For example, a planet in the sign of Cancer has the Moon as its dispositor, since the Moon rules Cancer.

A planet is the Final Dispositor when the rulership of all other planets can be traced back to that planet. It is always in its own sign, and has a lot of influence in the chart.

Mutual Reception is when two planets are in a sign that the other rules. For example, Venus in Scorpio and Mars in Taurus.

Each sign is said to be "ruled" by a planet. The ruling planet is able to express its purest energy in the sign(s) that it rules, and as such the ruling planet is very strong when in its sign of rulership.