Metamerism is defined as when two or more samples appear to be a good match under one set of conditions but fail to match under another set of conditions.
Note: that we use the general term 'conditions', because metamerism can take many forms:

• Illuminant Metamerism

• Observer Metamerism - we all view colours slightly differently

• Geometric Metamerism - colours viewed at different angles appear to be different

If we view an object under illuminant A, it tends to look yellower or redder. Viewing the same object under illuminant D65 will tend to make it look bluer, as shown in the example of these two greens:

Viewed under illuminant A

Viewed under illuminant D65

This leads us to consider the problem of colour matching. If we match a colour under one illuminant, will it still match the pattern under a different one ? Often the answer is NO! 

This is a phenomenon called Metamerism. The most frequent cause of metamerism is matching colours on different pigment combinations.