noun
A tied knot, often in a string, rope, or cord; can refer to the physical knot itself or the act of tying it.
ひもの結い目が固くてほどけない。
The knot in the string is tight and won't come undone.
結い目をしっかりと締めてください。
Please tighten the knot firmly.
結び目 is the more common word for 'knot' in everyday use. 結い目 is less frequent and may carry a slightly more traditional or literary nuance.
Compound of 結い (yui, 'tying', from verb 結う) and 目 (me, 'point, spot'). The exact historical derivation is uncertain, but the word transparently means 'tying point'.