Final answer:
The British Proclamation of 1763 halted colonial westward expansion by forbidding settlement west of the Appalachian Mountains, aimed at preventing conflict and reducing defense costs after the Seven Years' War, but was viewed by colonists as tyrannical.
Step-by-step explanation:
The law or proclamation from Britain that halted westward expansion by the colonists was the Proclamation of 1763. This proclamation forbade colonists from settling west of a line drawn along the Appalachian Mountains, which was known as the Proclamation Line. The British government issued this proclamation to prevent conflict with Native American tribes in the Ohio Valley and to avoid the costs associated with defending new territories, especially after accruing significant debt from the Seven Years' War. Despite British intentions to ease tensions, many colonists viewed the proclamation as an act of tyranny and continued to push westward, disregarding the restriction.