When you get nicotine it will be dissolved in another fluid. Typically in PG or VG or a mix of the 2. Thats a nicotine solution.
Their strength depends on how much Nic is dissolved in there -usually specified as milligrams of nic per milliliter of fluid.
The strongest is 100mg/ml. Thats quite dangerous so its usually diluted further like 60, 48, 36 mg/ml.
If you make
juice with Nic, thats a Nic solution too usually from 3mg/ml up to 30 mg/ml or so.
Nicotine thats not in solution would be 100% nic or about 1000mg/ml and that stuff is so dangerous its handled by lab techs in rubber suits and respirators. They'll dilute it down to 10% ... 100mg/ml or less before its sent to you.
Since we don't deal with the pure stuff you may see people here say Nicotine when they are referring to a nicotine solution.