Definition
A stage in the curing or drying of an adhesive, sealant, paint, or coating in which the material is no longer wet but remains sticky to the touch. In this state the surface will adhere to a finger or mating part with light pressure but does not transfer material or leave a wet film.
Plain English
Sticky, but not wet. The material has dried enough that it doesn't come off on your finger, but it still grabs when you press something against it.
Context Anchor
Seen in airframe maintenance instructions for bonding, sealing, fabric covering, painting, and coating work.
Derivation
From the older English word 'tack,' meaning a light stickiness or hold. The adjective form 'tacky' simply means 'having tack.' Knowing this helps because in maintenance manuals 'tack' and 'tacky' describe a specific working stage of the material, not a permanent condition.
Why Pilots Care
Many adhesives and sealants only bond correctly when joined at the tacky stage. Mating surfaces too early traps solvents; mating too late means the material has cured past the point where it will stick. Reaching the right tack window is often what makes the joint hold.
Intuition Check
Tacky does not mean cheap, poorly made, or in bad taste here. In maintenance, it means slightly sticky and partly set.
Example Sentence 1
Allow the contact cement to dry until tacky before pressing the two surfaces together.
Example Sentence 2
After the primer dried to a tacky state, the painter lightly sanded it before applying the color coat.