The definition of grain

The term grain refers to:

1. The general direction of the fibres or wood elements relative to the length of the piece; and

2. The direction, size, arrangement, appearance, or quality of the fibres in wood or timber.

It is not sufficiently specific to fully describe particular fibre arrangements and more detailed terms such as: straight grain, sloping grain, irregular grain, wavy grain, etc., are used.

Reference:

NAFI Timber Manual: Timber Datafile P1 – Timber Species and Properties.