Definition for Alkaloids
From Biology Forums Dictionary
1) Alkaloids are a group of naturally occurring chemical compounds that contain mostly basic nitrogen atoms. This group also includes some related compounds with neutral and even weakly acidic properties. Also some synthetic compounds of similar structure are attributed to alkaloids. In addition to carbon, hydrogen and nitrogen, alkaloids may also contain oxygen, sulfur and more rarely other elements such as chlorine, bromine, and phosphorus.
2) A group of structurally related secondary metabolites that all contain nitrogen and usually have a cyclic, ringlike structure.