HTML Attribute to Define Inline Styles:
The HTML attribute used to define the inline styles is the 'style' attribute. It is used to apply specific styles to individual HTML elements, allowing for customization of various aspects such as font size, color, background, margins, and more.

Explanation:
Inline styles are a way to apply unique styles directly to HTML elements, without the need for external CSS files or internal style sheets. The 'style' attribute is used to define these inline styles. Let's break down the usage of the 'style' attribute:

Syntax:
The 'style' attribute is added to an HTML element as an attribute. It is written within the opening tag of the element and consists of one or more style declarations.

Example:
Consider the following HTML code snippet:

```html

```

In the above example, the 'style' attribute is applied to the paragraph element (). Within the attribute value, we can define multiple style declarations separated by a semicolon (;). Each declaration consists of a property and a corresponding value, separated by a colon (:).

Properties and Values:
The 'style' attribute allows for a wide range of properties and values to be used. Some common examples include:

- color: Specifies the text color.
- font-size: Sets the font size.
- background-color: Defines the background color.
- margin: Sets the margin around the element.
- padding: Defines the padding around the element.

Multiple Style Declarations:
Multiple style declarations can be applied within a single 'style' attribute. Each declaration is separated by a semicolon (;). For example:

```html

```

In the above example, three style declarations are applied to the