ー

• (emphatic) 〰, 〜

Said to be from the right side of 引.

ー 

1. The long-vowel mark, which marks a long vowel in katakana (and less commonly in hiragana) when placed after a syllable.

• This mark appears as a horizontal line in horizontal writing (横書き) and as a vertical line in vertical writing (縦書き).
  • タクシー (takushī, “taxi”) when written vertically appears as:

    タクシー

• The Japanese names are 長音符 (chōonpu), 音引き (onbiki) or 棒引き (bōbiki)

• Chōonpu on Wikipedia.Wikipedia
• 長音符 on the Japanese Wikipedia.Wikipedia ^ja