Why does doing \\ in LaTeX causes Undefined control sequence error [closed]

Question

I have a weird LaTeX error here. It seems as if the newline (\\) command is the culprit of a compilation error. The error shows up at the ending line of the .tex document: the \end{document} line, stating that this line has the undefined control sequence.

This code, though, is actually what causes the error. This will compile (formatted incorrectly):

\defn{Method of Moments Estimator (MOME)}{a} 
Properties of MOME:

But this won't compile (formatted correctly):

\defn{Method of Moments Estimator (MOME)}{a} \\
Properties of MOME:

The only difference is the \\ at the end of the line. I have no idea why this should not work. For reference, this is the \defn command that I made:

\newcommand{\defn}[2]{
\tab
\textit{\textbf{Definition }}
\textbf{#1}
\textbf{:}
\text{#2}
}

And the \tab command:

\newcommand{\tab}{\text{    } \text{    } \text{    }}

I would greatly appreciate any help on this matter. Thank you to all who contribute.