The characteristic of life that is definitively defined as the ability of living things to produce their own kind is reproduction.

Here's why:

* Reproduction is essential for the continuation of a species. Without reproduction, a species would eventually die out.

* Reproduction ensures genetic diversity. Offspring are not exact copies of their parents, leading to variations that help a species adapt to changing environments.

* Other characteristics of life, while important, are not solely defining of life. For example, growth can occur in non-living things, and adaptation can be seen in systems that are not technically alive.

Let me know if you have any other questions!