What term is used for natural or synthetic hair added to the base for length or volume?

The term that best describes natural or synthetic hair added to the base for length or volume is "Extensions." Extensions are specifically designed to blend with the existing hair, providing a seamless look while adding both length and volume. They can come in various forms, including clip-ins, tape-ins, or wefts, and are applied to enhance the aesthetics of a person's hairstyle.

While "Hair Additions" might seem like a reasonable alternative, it is not as widely recognized in the industry as "Extensions." Wigs are typically full coverings for the entire head, not just additions for volume or length, and hairpieces generally refer to smaller sections of hair, like toppers, used to cover specific areas rather than adding volume or length across the entire head. Thus, "Extensions" is the most appropriate term for this practice.