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”).