noun

• (sewing) A loose seam used to temporarily fasten pieces of cloth.
• (nautical) The lower corner on the leading edge of a sail relative to the direction of the wind.
• (nautical) A course or heading that enables a sailing vessel to head upwind.
• (figurative) A direction or course of action, especially a new one; a method or approach to solving a problem.
  "So stoutly held to tack by those near North-wales men;"

verb

• (transitive) To nail (something) with a tack (small nail with a flat head).
• (nautical) To maneuver a sailing vessel so that its bow turns through the wind, i.e. the wind changes from one side of the vessel to the other.
• (intransitive, nautical) To sail to windward using a series of alternate tacks across the wind.
• (slang, obsolete) To join in wedlock.

noun

• (obsolete) A peculiar flavour or taint.
  "a musty tack"

noun

• (colloquial) That which is tacky; something cheap and gaudy.
  "For souvenirs – mostly outright tack and ethnicky textiles – try your bargaining skills at the shops and stalls on Binjiang Luand Zhengyang Jie, or the nightly street market spreading for about a block either side of Shanhu Bridge along Zhongshan Lu."