A small double cup ّ like a curved w or a 3 on its back, written in Arabic above a consonant to indicate it is doubled. (The D in shaddah would be an example.) When the vowel is kasrah the shaddah is still written above, with the kasrah below it instead of below the letter as it usually is. (The name may also be written shadda: see ta marbuta for why.)
One odd marginal use is the following. In Zaydun yaktubu 'Zayd is writing', the N of the article disappears in pronunciation (in Classical Arabic) but causes nasalization and lengthening of the following Y, which then takes shaddah.
In Unicode: shaddah is &1617;. Here are examples with the consonant د d: