Tack or tackiness in the context of material behaviour is associated with stickiness and may result from adhesive forces between two materials in contact or cohesive forces in a material bridging two substrates.

For pressure-sensitive adhesives such as tapes and labels, tack is defined as the
ability to form an adhesive bond to a substrate under slight pressure and brief
contact and is an essential requirement for such products.

For other materials and applications, tack may be an unwanted property, for example  bone cement must be tack-free according to the International Standards Organisation (ISO) 5833 to allow the user to shape and apply the cement without the need of gloves or application aids.