What term describes any changes made by a tailor to improve the fit of a garment?

The term that best describes any changes made by a tailor to improve the fit of a garment is "alteration." Alterations specifically refer to the process of altering a garment's dimensions to ensure a better fit for the wearer. This can involve shortening or lengthening sleeves, adjusting the waistline, or taking in the sides, among other modifications to the garment's structure.

While "modification" and "adjustment" might seem relevant, they are broader terms that can apply to a wide range of changes not specific to improving fit. Modification could refer to changes made for style or functionality, while adjustment might deal with minor tweaks rather than the thorough fitting adjustments that alterations encompass. "Tailoring" refers to the overall craft of making garments custom-fitted for individuals, which includes alterations but is not limited to the specific changes made for fit. Thus, alteration is the most precise term in this context.