Ilex opaca. Species name refers to the comparative dullness (opacity) of the leaf surface.

In your closeup you are showing two varieties. American on the right (opaca) and what looks like a Blue holly (x meserveae) on the left.

Yes - consistent with this in the original post it was said

In the photo of the two branches, you can see how light-green the leaves are compared to a darker Holly that is nearby

Thanks for the input! Below is a close-up photo of the leaves.

I thought the darker leaf on the left was American Holly. I'm surprised to hear that it might be Blue Holly, as the bush I cut it from dates to at least the mid 1970's, when our property was a 1/4 lot with a small broken-down ranch house. Difficult to believe the previous owner took the time to purchase and plant a Blue Holly.

Kathy Meserve created her blue hollies between '63 and '73. So it's possible. Usually English holly leaves are larger than what is pictured here, but they are variable.

Blue hollies are hybrids of aquifolium (and rugosa), so share many characteristics of aquifolium.

For clarity you might say 'the holly pictured on the left' or 'the holly pictured on the right' instead of 'nearby'.