ー - Wiktionary, the free dictionary

From Wiktionary, the free dictionary

See also: 一

• (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).
  • For example, タクシー (takushī, “taxi”) is written vertically as:

    タクシー

• The names for the symbol are 長音符 (chōonpu, literally “long sound mark”), 音引き (onbiki, literally “sound lengthener”) or 棒引き (bōbiki, literally “stick lengthener”).