If A. have Black Acre and C. have White Acre, and A. has a way over White Acre to Black Acre, and then purchases White Acre, the way will be extinct; and if A. afterwards enfeoff C. of White Acre without excepting the road, it is gone.
Source: wiktionary
For example, a gift of Blackacre today, when Blackacre is worth $100,000, will freeze the value of Blackacre at $100,000 forever when determining the value of the transferor's transfer tax base, assuming Blackacre will not be brought back into the transferor's estate under I.R.C. ยง 2036, 2037, or 2038 because the grantor has retained some right or power over Blackacre.
Source: wiktionary