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perpetuity
2: the quality or state of being perpetual <bequeathed to them in perpetuity>
3 a : the condition of an estate limited so that it will not take effect or vest within the period fixed by law
b : an estate so limited
[M-W's Col. Dic.]

Could you please help me with the boldface parts, especially 3a?
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Hi;
This is a specialized legal definition.

Here is an example:
By law, an estate (property, money, business interests) will pass to a child when he becomes a legal adult. But the parents can put a limit on their estate so their child will not inherit the money or property until he is much older.

Each country has different laws regarding inheritance and perpetuity of trusts. Here is a recent legal ruling in the UK regarding perpetuity limitations - LINK
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Help, please.
 AlpheccaStars's reply was promoted to an answer.