There are plenty of times when using LaTeX, that one uses bold fonts. Usually, these bold fonts are just the typical English letters from the Latin alphabet. How about Greek letters though and how abnout other synbols? 

Although that there are couple of solutions, the most simple (according to my opinion) is to use the \boldymbol command, which is already included in the amsmath package. 

A minimum working example is: 

\documentclass{article}
\usepackage{amsmath}

\begin{document}
Let $\boldsymbol\alpha$ be a vector.  Call it $\boldsymbol\beta$.
\end{document}

And you are done! 

P.S. Example taken from: https://math.berkeley.edu/~vojta/tex/samp-l/boldgreek