terminology - What is the difference between these two depictions of chromosome?

I understand that this is a single chromatid, but would this be considered a chromosome? Also before mitosis, the chromosomes appear as single chromatids but during interphase they replicate to form sister chromatids which are attached by a centromere and are also collectively known as a chromosome. I do not understand the difference between the two of them. Are they both to be termed chromosomes?