What is a Rainbow baby?

Hi ladies, I have a question about what is considered a Rainbow baby.. let me explain...

Last July my husband and I were expecting a little girl. At our 20 week scan, she was diagnosed with Trisomy 13. We were told she had numerous health problems that would prevent her from living past a couple days. After a third opinion, we decided for the sake of her as well as our family that it would be best to terminate the pregnancy. It was the hardest decision of our lives. We have now reached the point where we would like to try again and I'm wondering if the next would be considered our rainbow?

Please understand that we made the decision that was best for our family. I'm not trying to be rude, but I just want answers for my question and not any input on how we should've handled the situation differently.

Thank you!