Niddah, Chapter 6, Mishnah 13. The closing mishnayos of this chapter deal with hairs: the two hairs that mark the transition from childhood to adulthood, and now, in our mishnah, hairs in other areas of halachah as well. The question here is a simple one that runs through all of them: how much growth does it take before a hair is halachically a hair?
The mishnah begins by listing the settings in which the question arises: "shtei se'aros ha'amuros baparah", the pair of hairs mentioned in connection with the cow. The reference is to the parah adumah, the red cow, whose coat must be red throughout. Should a pair of hairs of some other color, black or white, be found on it, the animal is unfit. Here, then, is one place where our question applies.
"Uvanega'im", and the hairs spoken of in the laws of nega'im. There too, the status of a person's tzaraas can depend on the color of the hairs found in the affected spot.
And beyond these, "veha'amuros bechol makom", the hairs mentioned anywhere else in halachah. Wherever hairs come up for discussion, the mishnah now supplies a measure.
The first opinion: "kedei lachof roshan le'ikaran", the hair must be long enough that its tip can be bent back to reach its root. That is, one folds the hair over on itself, and if the top can be brought around to touch the bottom, it counts as a hair. This is the view of Rabbi Yishmael.
Rabbi Elazar offers a second measure: "kedei liknos bitzipo'ren", long enough that one could take hold of it with a fingernail. If it can be grasped that way it qualifies as a hair; if it is shorter than that, it has no halachic status as a hair for these purposes.
Rabbi Akiva supplies a third measure: "kedei sheyihyu nitalos bezug", the growth must be sufficient that a pair of shears could take hold of it and clip it off.
The mishnah therefore does not leave the word "hair" undefined. Whether we are examining a red cow for disqualifying hairs, inspecting a nega, or asking about hairs in any other context, three measures are offered, each one a practical test of whether the growth has reached the point where halachah calls it a hair.