the planet that rules a given sign or house
the planet ruling the rising sign (Ascendant) is said to be the Ruler of the chart. Thus if you have Cancer (or Gemini) rising, the Moon (or Mercury) is the ruler of your chart.
A planet is said to rule a house when it rules the sign on the cusp of that house.
Each Sign of the Zodiac and each of the 12 Houses are associated with a specific Planetary energy, and is said to be ruled by that Planet. The Planet that rules the sign on the Ascendant is called the Chart Ruler.
The assigned planet to a sign based upon its affinity with the sign's qualities. (Also known as Lord of the sign.) For example, Pluto is the Ruler of Scorpio.
Each sign of the zodiac and each of the 12 houses is associated with a specific planet, and is said to be ruled by that planet. Every sign and every house in a chart has a planetary ruler. More info on Rulers