Ink tack is the stickiness of an ink. Printing inks for offset lithography have to be tacky enough to adhere to the blanket, and then the paper, but not so tacky that they won’t let go. Here’s a description of the issue from Paper, Film & Foil Converter. When printing more than one color, the second (and subsequent) ink must have less tack than the one it’s being laid over or it will not trap.  On press, tack increases both as speed increases, and as ink film thickness is lowered. As temperature increases, tack decreases. Thus tack levels for web offset offset inks will be around ⅔ of what they are for sheet-fed offset. The amount of tack can be adjusted by adding solvent, such as linseed oil, to the ink.

The video at Ink making shows ink being tested for tack.