Flakey is the term applied to aggregate or chippings that are flat and thin with respect to their length or width,

Aggregate particles are said to be flakey when their thickness is less than 0.6 of their mean size.

The flakiness index is found by expressing the weight of the flakey aggregate as a percentage of the aggregate tested.

This is done by grading the size fractions, obtained from a normal grading aggregate, in special sieves for testing flakiness.
These sieves have elongated rather than square apertures and will allow aggregate particles to pass that have a dimension less than the normal specified size, i.e. 0.6 of the normal size.
This grading process is normally performed by hand because flakey chippings tend to 'lie' on the sieve surface rather than fall through the aperture.

There are a number of material and aggregate specifications that have a maximum amount of flakey material allowed, e.g. surface dressing chippings.
Flakey aggregate has less strength than cubical aggregate, and does not create the dense matrix that well graded cubicle aggregate is able to do, and it will provide less texture when used in surface dressing.

E.g. Granular sub-base with a high proportion of flakey aggregate tends to segregate and be difficult to compact, although performing a normal aggregate grading test will show it conforms to specification.

Flakey chippings do not create the surface texture that a cubicle or angular chipping is able to produce.