Punnett Square is devised by 

a table which helps in determining the genotype and phenotype of offspring

the cross happening in the F1 generation offspring of parents differing in one trait only 

as cross in individuals involving only one pair of contrasting traits 

a cross between two individuals with two observed traits that are controlled by two distinct genes 

an experimental cross of an individual organism of dominant, phenotype, but unknown genotype and an organism with a homozygous recessive genotype and  phenotype.

a chart that shows the presence or absence of a trait within a family across generations

if the trait is dominant, one of the parents must have the trait. 

if both parents are unaffected all offspring must be unaffected 

if both parents are unaffected and an offspring is affected 

"skipping" generations