Contamination
Contamination is the unintended presence of something harmful in food: bacteria, mould, pests, chemical residue, or a foreign object. It usually results from poor hygiene, storage, or handling rather than deliberate action by the seller.
Adulteration
The Food Safety and Standards Act, 2006 defines an 'adulterant' as any material that is or could be used to make food unsafe, sub-standard, misbranded, or to affect its nature, substance or quality. Adulteration is often deliberate, for example mixing a cheaper substance into milk, oil or spices, but the Act also uses the term for food that fails safety or quality requirements for other reasons.
In practice: a fly in your food is usually contamination; chalk powder mixed into sugar is adulteration.
Why the distinction matters for a report
The right official channel and evidence needed can differ depending on which problem you are reporting. Describe exactly what you observed (taste, smell, appearance, foreign matter) rather than guessing a label, so moderators and officials can route it correctly.