Vortex line

Japanese: 渦線 - うずせん

...Free vortex sheets are generally unstable and transition into complex, turbulent vortex motion (turbulence) with large and small vortices.
[Vortex line]
Just as the concept of streamlines is used to help understand the flow of a fluid, vortex lines are used to explain the motion of a vortex. A streamline is a curve whose tangent direction indicates the direction of the flow at that point, whereas a vortex line is a curve whose tangent direction coincides with the direction of vorticity at that point. …

From [Flow]

… Twice the angular velocity Ω, ω = 2Ω, is called vorticity, and a flow without such rotation is called a flow without vortices, while a flow in which such rotation is distributed is called a flow with vortices. In particular, the curve created by connecting the rotation axes of fluid particles one after another is called a vortex line. Also, the tube created by the vortex lines that pass through each point of a small closed curve taken in a flow with vortices is called a vortex tube. …

*Some of the terminology explanations that mention "vortex lines" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…自由な渦層は一般に不安定であって大小の渦が複雑に乱れた渦運動(乱流)に移行する。
［渦線］
　流れのようすをわかりやすくするのに流線という概念が用いられるのと同じように，渦の運動には渦線というものが用いられる。流線は，接線の方向がその点における流れの方向を示す曲線であるが，渦線は，接線の方向がその点における渦度の方向と一致するような曲線である。…