In general, the concept that some quantitative aspect of an entity remains unchanged despite a transformation in its appearance. Piaget devised a series of tasks to test children’s understanding of conservation in a variety of entity domains (e.g., mass, number, volume, weight). For example, in conservation of mass tasks, children are tested on whether or not they understand that two identical balls of clay will have the same amount (mass) even if one of them is rolled out into a sausage. In terms of number, if two rows containing equal numbers of counters are presented in one-to-one alignment, and then the counters in one row are moved closer together, the number of counters in the two rows remains the same.
See Décalage, Object reversal test