Boyd’s Bible Dictionary:

(lilies). Variously construed as a melody, bridal-song, and musical instrument (Psa. 45; 69; 80 titles). In the latter, eduth, “testimony,” is added.

Concise Bible Dictionary:

A word in the headings of Psalm 45 and Psalm 69, and of Psalm 80 with the word EDUTH, “a testimony,” added. The first Hebrew word signifies “lilies.” Gesenius refers it to the form of the instruments as resembling lilies. Fürst, as the name of one of the musical choirs. The LXX has “for alternate [strains].” In the heading of Psalm 60 is a similar word: SHUSHAN-EDUTH, “the lily of testimony.”

Jackson’s Dictionary of Scripture Proper Names: