Chromosome map
A chromosome (one of the microscopically visible carriers of the genetic material DNA) with its centromere (the "waist" of the chromosome) located quite near one end of the chromosome.
The "acro-" in acrocentric comes from the Greek "akron" meaning "the end." Humans have 5 pairs of acrocentric chromosomes.
Down syndrome (trisomy 21) is due to an extra acrocentric chromosome, namely chromosome 21.