The function $\lceil \cdot \rceil$ is the _ceiling function_ or the _least integer function_. This function takes a real number $x$ as an input, then gives the least integer greater than or equal to $x$ as output. For example:
$$ \lceil 1 \rceil = 1, \qquad \lceil \pi \rceil = 4, \qquad \lceil -\tfrac{3}{2} \rceil = -1, \qquad\text{and}\qquad \lceil 1.00000000001\rceil = 2.$$