Tenuto (Italian, past participle of tenere, "to hold") is a direction used in musical notation. It is one of the earliest directions to appear in music notation. Notker of St. Gall (c.840 - 912) discusses the use of the letter t in plainsong notation as meaning trahere vel tenere debere in one of his letters.
The precise meaning of tenuto is contextual: it can mean either hold the note in question its full length (or longer, with slight rubato), or play the note slightly louder. In other words, the tenuto mark may alter either the dynamic or the duration of a note. Either way, the marking indicates that a note should receive emphasis.
The mark's meaning may be affected when it appears in conjunction with other articulations. When it appears with a staccato dot, it means non legato or detached. When it appears with an accent mark, because the accent indicates dynamics, the tenuto means full or extra duration.
Tenuto is notated three ways: