就是音乐术语tenuto。
以下为音乐字典中的标准解释
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
标示tenuto的根据不同情况可能是稍作强调，或者保持音符的完整时值或者更长。这里的强调不一定是特指加强音量，是给予特殊的注意（比如让其听起来凸显出来等）
一般情况下（包括给出的图片）都可以理解为保持完整时值，稍作强调的意思
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