When two words act as a single modifier or adjective for noun is this called compound modifier?

Yes, when two words act as a single modifier or adjective for a noun, it is called a compound modifier.

Examples:

* Long-term investment

* High-speed internet

* State-of-the-art technology

* World-renowned artist

Rules for Compound Modifiers:

* Hyphenate compound modifiers when they precede the noun they modify:

* *long-term* investment

* *high-speed* internet

* Do not hyphenate compound modifiers when they follow the noun they modify:

* The investment is *long term*.

* The internet connection is *high speed*.

* If the first word is an adverb ending in -ly, do not hyphenate:

* *newly* acquired skills

Note: There are some exceptions to these rules, so it's always a good idea to check a dictionary or style guide if you're unsure.