How to give tabs or indent to symbol representation

Question

I need to give indent or tab to each symbol I tried \quad, \tab, \indent but doesn't work.

\begin{equation}
\begin{split}
Where,\\
&$u_{i}=$ Horizontal displacement
&$v_{\mathrm{i}}=$ Vertical displacement\\
&$\varphi_{i}=$ Rotation Angle\\
\end{split}
\end{equation}

Answer 1

Something like this is it sufficient and is corresponds to your request?

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb}
\usepackage{newtxtext} % optional package for text font in Times New Roman
\begin{document}
Where,
\begin{equation*}
\begin{split}
u_{i} &= \text{Horizontal displacement}\\
v_{i} & = \text{Vertical displacement}\\
\varphi_{i} & = \text{Rotation Angle}
\end{split}
\end{equation*}
\end{document}

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb}
\usepackage{newtxtext} % optional package for text font in Times New Roman
\usepackage{newtxmath} % optional package for math-font Times New Roman
\begin{document}
Where,
\begin{equation*}
\begin{split}
u_{i} &= \text{Horizontal displacement}\\
v_{i} & = \text{Vertical displacement}\\
\varphi_{i} & = \text{Rotation Angle}
\end{split}
\end{equation*}
\end{document}

Answer 2

Here's a solution that employs an array environment.

\documentclass{article}

\usepackage{array}            % for \newcolumntype macro
\newcolumntype{L}{>{$}l<{$}}  % (cancel) automatic math mode

\begin{document}
\noindent
Where,\\
\hspace*{1.5cm}%
$\begin{array}{@{} r @{{}={}} L @{}} % 1st col. in math mode, 2nd col. in text mode
  u_{i}          & Horizontal displacement \\
  v_{\mathrm{i}} & Vertical displacement\\ % why '\mathrm{i}' ??
  \varphi_{i}    & Rotation Angle 
\end{array}$

\end{document}