How to Define a Command for Italic Text in LaTeX

If a command intended to italicize text fails, the usual causes are a missing argument declaration or using a text-formatting command in math mode. For ordinary prose, define a command with \textit{#1} and declare one argument with [1].

Define a command with one text argument

Use this form in the preamble, before \begin{document}:

\newcommand{\myital}[1]{\textit{#1}}

Here, \myital is the command name, [1] says it takes one argument, and #1 marks where that argument goes in the replacement text. Use it like this:

This word is \myital{emphasized}.

The braces delimit the argument. The result is ordinary text with “emphasized” set in italics.

Complete working example

This minimal document needs no extra package:

\documentclass{article}

\newcommand{\myital}[1]{\textit{#1}}

\begin{document}
This word is \myital{emphasized}.

The command also works with a longer phrase: \myital{italic text in a sentence}.
\end{document}

The definition belongs in the preamble so it is available throughout the document. The command can be used more than once, with a different braced argument each time.

Why a definition may fail

The argument count is missing. If the replacement text contains #1, the definition must declare at least one argument. This definition is incomplete and causes an error:

\newcommand{\myital}{\textit{#1}}

Write \newcommand{\myital}[1]{\textit{#1}} instead. For two arguments, declare [2] and use #1 and #2 in the replacement text.

The command is being used in math mode. \textit formats text; it is not the usual way to italicize a mathematical symbol. For a math symbol or expression, use a math-mode command:

$\mathit{AB}$

For ordinary words inside a formula, use \text{...} from the amsmath package. For example, \usepackage{amsmath} in the preamble enables $x \text{ is positive}$. If you specifically want those words italic, you can use \text{\textit{positive}}.

The command is used without its argument. After defining a one-argument command, supply the argument in braces: \myital{word}. A bare \myital does not provide the intended text.

The name has already been defined. \newcommand reports an error if the command name already exists. Choose a new name, or use \renewcommand only when you deliberately intend to replace an existing definition.

Italics and emphasis are not quite the same

For a reusable formatting command, \textit{...} is explicit. In prose, \emph{...} is often better when the goal is emphasis rather than a fixed italic appearance: LaTeX can change the emphasis style when it occurs inside already-emphasized text. If the request is specifically for italic text, \textit is appropriate.

If a definition still fails, check the exact error message and the mode where the command is called. A short document containing only the preamble definition and one use, like the example above, makes it easier to isolate the problem.