A more thorough explanation:

Measuring life is a term used in the legal concept of the rule against perpetuities. It refers to the last beneficiary who was alive at the time of the testator's death and holds a preceding interest. This person is used to determine whether an interest will vest under the rule against perpetuities.

For example, let's say a person creates a trust that will last for 100 years after their death. The trust states that the assets will be distributed to the testator's grandchildren when the last grandchild reaches the age of 30. In this case, the measuring life would be the last grandchild to reach the age of 30. If this grandchild dies before reaching the age of 30, then the trust assets will be distributed to the other grandchildren who meet the age requirement.

Another example would be a will that leaves property to a charity, but only if the charity is still in existence 50 years after the testator's death. The measuring life in this case would be the person who is alive at the time of the testator's death and who is expected to live the longest. If the charity ceases to exist before the end of the measuring life's lifetime, then the property will not go to the charity.

Overall, measuring life is an important concept in estate planning and helps ensure that interests in property are not tied up for an unreasonable amount of time.