It is a situation in which a small change in price causes an infinite change in quantity demanded. A small rise in price on the part of the seller reduces the demand to zero. A small reduction in price, on the other hand, leads to an infinite increase in demand. In real life, no commodity has perfectly elastic demand. The numerical value of price elasticity is infinity $(e_p = \infty)$ and demand curve becomes parallel to $X$ axis as shown in the given diagram.