Metamerism is the phenomenon wherein two colored samples will appear to be of the same shade under one light source but will appear to be different shades under a second source. For example your slacks and socks might match well at home under incandescent light but may not be a suitable match under the floursecent lighting at the office.
Physical samples will only match under all sources if their spectral reflectance curves have precisely the same shape. When the samples' reflectance curves have different shapes, typically crossing at least three times, then they are likely a metameric pair.
The two different colors have the same color appearance under one light source (metameric match) but look different under another light source (metameric mismatch).
Metamerism is the difference of the reflection curves of two colors which look the same under a given form of lighting (e.g. electric bulb light, daylight). Only colors which display the same remission curves (M=0) will be perceived as the being the same color under various light sources.