This word "faithfulness" appears in the Old Testament of King James version of the bible 19 times.
I believe that faithfulness has two sides to it. God has declared that He will be faithful to us. Now our part is to be faithful to Him. We have to outright declare His faithfulness as we will see in the word of God.
Here is the Hebrew word of faithfulness: hnwma 'emuwnah, em-oo-naw'. It literally means firmness; figuratively security; morally fidelity:--faith(-ful, -ly, -ness, (man)), set office, stability, steady, truly, truth, verily.
Here are two references for you to read and meditate this week.
Ps 36:5 ¶ Thy mercy, O LORD, [is] in the heavens; [and] thy faithfulness [reacheth] unto the clouds.
Ps 40:10 I have not hid thy righteousness within my heart; I have declared thy faithfulness and thy salvation: I have not concealed thy lovingkindness and thy truth from the great congregation.