Knowledge and a spiritual witness given by the Holy Ghost. A testimony can also be an official or legal declaration of what a person perceives as truth (D&C 102: 26).

I know that my redeemer liveth, Job 19: 25-26. Be not ashamed of the testimony of our Lord, 2 Tim. 1: 8. The testimony of Jesus is the spirit of prophecy, Rev. 19: 10. Stand as witnesses of God at all times, Mosiah 18: 9. There was no way to reclaim the people save by bearing down in pure testimony against them, Alma 4: 19-20. I have all things as a testimony that these things are true, Alma 30: 41-44. Ye receive no witness until after the trial of your faith, Ether 12: 6. Did I not speak peace to your mind? What greater witness can you have than from God? D&C 6: 22-23. And now, after the many testimonies which have been given of him, this is the testimony which we give of him, D&C 76: 22-24. I sent you out to testify and warn the people, D&C 88: 81-82. The testators are now dead, and their testament is in force, D&C 135: 4-5. Enoch beheld angels bearing testimony of the Father and Son, Moses 7: 27. Though I was hated and persecuted for saying that I had seen a vision, yet it was true, JS-H 1: 24-25.