"But you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth.”
The word power is the Greek word, dunamis (doo'-nam-is). It means: force (literally or figuratively); specially, miraculous power (usually by implication, a miracle itself), and can be translated as: ability, abundance, meaning, might(-ily, -y, -y deed), (worker of) miracle(-s), power, strength, violence, mighty (wonderful) work.
It is from the Greek word, dunamai (doo'-nam-ahee), meaning of uncertain affinity; to be able or possible: be able, be possible, be of power.
Jesus' call to be witnesses to the ends of the earth wasn't given alone; it was given along with the promise of the Holy Spirit. The Holy Spirit's power is the means by which this call can be accomplished.
*Strong, James, LL.D., S.T.D., The New Strong’s Expanded Exhaustive Concordance of the Bible, Red Letter Edition (Expanded With the Best of Vine’s Dictionary of Old & New Testament Words) (Nashville: Thomas Nelson, 2010).