What is $pt$ in Hartshorne Ch2 Example 6.6.1 (Calculating $Cl(Q)$ which $Q$ is the surface $xy=zw$ in $\mathbb{P}^3$)

Question

I'm reading Hartshorne Ch2 Example 6.6.1. $Q$ is the quadric surface $xy=zw$ in $\mathbb{P}^3$. The book says Let $Y=pt \times \mathbb{P}^1.$ Then $Q-Y=A^1 \times \mathbb{P}^1$.

My question is: What does $pt$ mean?

Thank you!