Irrational numbers are numbers that cannot be expressed as a simple fraction; they have non-repeating, non-terminating decimal expansions. Among the numbers you provided:
- π (pi): This is an irrational number.
- 19/74: This is a rational number because it can be expressed as a fraction of two integers.
- 64: This is a rational number because it can be expressed as 64/1.
- 19: This is also a rational number because it can be expressed as 19/1.
Therefore, the only irrational number in the list is π (pi).