What is Proviso For Cesser?

Legal Definition
Where terms for years are raised by settlement, it is usual to introduce a proviso that they shall cease when the trusts end. This proviso generally expresses three events:

(1) The trusts never arising;

(2) their becoming unnecessary or incapable of taking effect;

(3) the performance of them. Sugd. Vend. (14th Ed.) 621-623.
-- Black's Law Dictionary