An alkaloid is a generic term for a molecule with a nitrogen base. Nicotine is a tobacco alkaloid among several others that include cotinine, nornicotine, anatabine and anabasine.
Nicotine is the main tobacco alkaloid. However, depending on the type of tobacco and the part of the tobacco leaf, a non-negligible percentage of other alkaloids can be found.
The nicotine used in e-liquids must be EU Pharmacopoeia grade in order to limit the presence of alkaloids that can be harmful to health. EU Pharmacopoeia grade complies with a standard that gives a threshold for nicotine impurities that includes alkaloids.