I'm pretty sure that it's a chromosome when it's a single straight line- l - and also when it's an X shape. So normally you have 46 chromosomes in a cell, l shaped. When your DNA replicates for mitosis or meiosis you get an X shaped chromosome- this is one chromosome with two chromatids, held together by the centromere, which is that dot at the middle of the X. Think of it like two identical pieces of string, each one being a chromatid, which are tied together in the middle. When the cell divides (for mitosis), the two chromatids are separated to from two chromosomes with are l shaped, one single strand. These two chromosomes formed from the two chromatids are totally identical.
Of course in meiosis it gets fancy and divides twice but that's basically it for the whole chromosome vs chromatid thing. My teacher says people always gets confused by that because textbooks talking about mitosis/meiosis always show chromosomes as X because they're in the middle of replicating, but in fact normally they aren't X shaped, just one strand.