What does the term "nail extension" refer to?

The term "nail extension" specifically refers to the process of applying artificial nails to extend the length of natural nails. This technique involves using materials such as acrylic, gel, or fiberglass to create a longer nail appearance than what is naturally achievable. Nail extensions can be a solution for those who want longer nails for cosmetic reasons or to create specific nail art designs.

In contrast, growing natural nails longer is a completely different concept that does not involve the use of artificial materials. The application of varnish on nails pertains to adding color and shine but does not physically extend the nail's length. Using gel to strengthen nails also focuses on enhancing the durability and appearance of natural nails rather than extending them. Thus, the correct answer accurately captures the essence of nail extensions as a method of enhancing and altering the nail length with artificial products.