In chemistry, a<unk> is a compound that is formed from a similar compound or a compound that can be imagined to arise from another  compound, if one atom is replaced with another atom or group of atoms. The latter definition is common in organic chemistry. In biochemistry, the word is used about compounds that at least theoretically can be formed from the precursor compound.
Chemical<unk>  may be used to facilitate analysis.  For example, melting point (MP) analysis can assist in identification of many organic compounds.  A crystalline<unk> may be prepared, such as a semicarbazone or 2,4-dinitrophenylhydrazone (derived from aldehydes/ketones), as a simple way of verifying the identity of the original compound, assuming that a table of<unk> MP values is available.  Prior to the advent of spectroscopic analysis, such methods were widely used.
Derivative 
Enthesis
Appendage
Formication
Anise
Staining
Splitting 
Amplification 