A chromatid forms one part of a chromosome after it has coalesced for the process of mitosis or meiosis. During either process, the word "chromosome" indicates a pair of two exactly identical ("sister") chromatids joined at the central point of each chromatid, called the centromere.
A chromosome, by definition, has exactly one centromere. Thus when sister chromatids are pulled apart by the mitotic spindle during anaphase -- that is, when the centromere joining two sister chromatids splits into two centromeres -- then each sister chromatid becomes an independent chromosome.
In non-gametic, non-dividing human cells, there are 23 pairs of chromosomes, thus 46 chromatids. When it is ready for mitotic cell division, each chromosome will replicate itself during the Synthesis phase within its life cycle, making a total of 92 chromatids before dividing, producing two cells each with 46 (remember, since the term "chromosome" is redefined,but still the number of chromosomes remains 46 despite the replication).
In gametic cell division (meiosis),the cell replicates once but divides twice so that 23 chromosomes are left in each of four egg or sperm cells. (23 chromosomes are each needed from the male and female to combine and make 46 chromosomes for the zygote.)