Border-spacing is given in Pixels.

Explanation:
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HTML provides various attributes and properties to control the layout and spacing of elements on a web page. One of these properties is "border-spacing", which is used to specify the space between the borders of adjacent table cells.

The "border-spacing" property is used in conjunction with the "border-collapse" property, which determines how the borders of adjacent cells are displayed. When "border-collapse" is set to "collapse", the value of "border-spacing" is ignored.

The "border-spacing" property accepts a length value, which can be specified in different units such as pixels, centimeters, millimeters, or inches. However, the correct answer for this question is pixels.

Pixels (px) is a unit of measurement commonly used in web development. It represents a single dot on a computer screen. The advantage of using pixels for specifying "border-spacing" is that it provides a consistent and predictable spacing regardless of the user's screen size or resolution.

Other units of measurement like centimeters (cm), millimeters (mm), or inches (in) are absolute units and may not provide the desired spacing on different devices or screens. Using pixel values ensures that the spacing remains consistent and visually appealing across different devices and screen sizes.

In conclusion, the correct unit of measurement for specifying "border-spacing" is pixels (px). This ensures consistent and predictable spacing between the borders of adjacent table cells on various devices and screen sizes.