The type of reproduction requiring the union of two specialized cells is referred to as sexual reproduction.

Here's why:

* Specialized cells: These are called gametes, specifically the sperm (male) and egg (female).

* Union: This process is known as fertilization, where the sperm and egg fuse to form a zygote.

* Zygote: The zygote contains genetic material from both parents, leading to offspring with a combination of traits.

Let me know if you'd like to know more about sexual reproduction!