Objective

is to represent bold text or symbol in LaTeX.

Example 1. Bold Text

Code

Hello! Welcome to the \textbf{Study Astrophysics}.

Result

Example 2. Bold inside an equation

Code

\mathbf{A} \cdot \mathbf{B} \equiv AB \cos\theta

Result

Example 3. Bold Symbol

Code

\boldsymbol{\lambda}

Result

Reference

https://www.sharelatex.com/learn/Bold,_italics_and_underlining
http://study-astrophysics.com/griffiths-01-01/