Cat & Pig on Pi Day, digital phone drawings, 3/14/19

"The number π is a mathematical constant. Originally defined as the ratio of a circle's circumference to its diameter, it now has various equivalent definitions and appears in many formulas in all areas of mathematics and physics. It is approximately equal to 3.14159. "
Wikipedia, as answered in my google search.

Pi - also the 16th letter of the Greek alphabet, 'p'.