Mergeable Heap
Definition
Mergeable heaps (also known as meldable heaps) are a class of heap that support an additional operation: merging two heaps into one.
We are considering a min mergeable heap. Alternatively, we could define a mergeable max heap with the operation MAXIMUM and EXTRACT-MAX.
Time complexity
The time complexity of the mergeable heaps’ operations, including the additional merge operation, will depend on the type of heap used.
For example, if using Fibonacci heaps, merging complexity will be $O(1)$ as you only need to concatenate the root lists of the two heaps. But, for binomial heaps, it would be $O(log n)$ as you’ll need to combine trees.