A relational symbol indicating "less than or approximately equal to" commonly used in analysis and inequalities.
Examples
Comparing growth rates of functions
n^2 \lesssim n^2 + n \text{ for large } n
Estimating physical quantities
\text{Mass of electron } m_e \lesssim 9.1093837 \times 10^{-31} \text{ kg}
Bounding mathematical expressions
|f(x)| \lesssim M\|x\| \text{ for some constant } M > 0