Laws of Time

The Laws of Time are a set of guidelines and laws guiding what a time traveller may do. As opposed to physical Laws, which are observations of constants of behavior found in nature (ie The Laws of Motion or Gravitation), the Laws of Time are rules or guidelines put in place to prevent massive changes to the primary timeline by time travellers.

They traditionally prevented Gallifrey's 'present' from interacting with its subjective past or future. (NA: Lungbarrow).

It is unknown how those law are (or were) enforced, or what the exact punishment for breaking them was. It is possible the effects of breaking them were serious enough that they would themselves serve as a deterrent.

The Doctor has at stated that he was "...Defender of the Laws of Time" in his seventh persona and eighth persona. (DW: Remembrance of the Daleks, EDA: Vampire Science)

The Laws of Time became weakened during the Second War in Heaven so that future events filtered back to influence interact with events prior to the war's outbreak. (EDA: Alien Bodies, The Taking of Planet 5)

Specific Laws of Time
One of the Laws of Time supposedly prevents Time Lords from meeting their previous selves, but this is broken several times in emergencies. (DW: The Three Doctors, The Five Doctors, BFA: The Sirens of Time)

One of the Laws of Time states that an object from a non-existent timeline be present in the current timeline. Cousin Justine of the Faction Paradox had a mask of such description. (EDA: Alien Bodies)

Learning "anything about future Gallifreyan history, causality could be damaged beyond conception...I'm breaking one of the major Laws of Time...It could be the third." -The Doctor (EDA: Alien Bodies)