Sempre
“Always”, maintaining.
What it means
Sempre means “always” and calls for maintaining an instruction over a longer stretch. Sempre piano means that the soft dynamic applies even where one might be tempted to become louder; sempre legato, that the binding is kept up throughout; sempre crescendo, that the intensification continues without interruption.
The word never stands alone but connects with the instruction it extends. It often stands precisely where the notated text would tempt a deviation — for instance with rising lines that call for a crescendo, or at points where the articulation could change.
The French equivalent is toujours, the German immer.
In the notation
The word stands together with the marking concerned, above the staff for tempo and performance instructions, below the staff for dynamics. It applies until a new marking. It often replaces a repetition of the same instruction in every bar and thereby keeps the notation clear.
Musical use
The word is used to fix an attitude over a longer section: a constant dynamic under a moving melody, a continuous articulation in an accompanying figure, an intensification extending over many bars. In orchestral music it serves to ensure that everyone involved keeps to the same instruction even when their parts run differently.
The French equivalent is toujours, the German immer. It reminds one that an articulation or volume is not to slacken, even as the music changes. Sempre crescendo means to keep becoming louder continuously. The instruction applies until a new marking replaces it, and is therefore always to be kept in view when reading.
Performance practice
Take the addition as a warning: it stands where the deviation suggests itself. Check especially rising lines and dense passages for whether you are unnoticedly becoming louder or faster. Keep the instruction up until the next marking. And mark for yourself the end point, so that you know where the restriction ends.