I am following along with a score of Beethoven's first symphony. I came across two abbreviations, arco. and ten. What do those stand for?
Arco (which is not an abbreviation) means to return to bowing after pizzicato (abbreviated pizz.) or col legno. Pizzicato means you pluck the strings with your fingers instead of using the bow, col legno using the wooden backside of the bow instead of the hairs.
Ten. is short for tenuto which means holding. In Beethoven it probably means you should hold the note for its full length instead of leaving a little gap between the notes. In later music (and also in Beethoven when played in a more romantic style) it also often means that you hold the note for longer than notated (using rubato) and/or play the note a bit louder. In any case it's a sort of "slow accent".