Chromosomes that look like X's are called replicated chromosomes or sister chromatids. The X shape appears during cell division (mitosis or meiosis) when DNA has been replicated, and each chromosome consists of two identical sister chromatids joined at a central region called the centromere. This X shape is a transient, iconic form visible under a microscope during the mitotic phase, not the usual shape of chromosomes in non-dividing cells, where they appear more as amorphous blobs or threads. In summary:

• The X-shape represents two sister chromatids joined at the centromere.
• This shape occurs during cell division (mitosis or meiosis).
• The term does not refer to a chromosome being called "X chromosome" based on its shape; rather, all replicated chromosomes can appear X-shaped.
• The centromere position determines the exact shape, which may sometimes look more like a V or Y depending on the chromosome.

So chromosomes that look like X's are replicated chromosomes visible during cell division, consisting of two sister chromatids joined together.