Knitting term
In knitting, an increase is the creation of one or more new stitches, which may be done by various methods that create distinctive effects in the fabric. Most knitting increases either lean towards the left or the right.
Methods of single increasing (knitting).
Finally, a large number of increases in a row is best done by casting on; examples include buttonholes, etc. Increases are typically placed on the right side of a project but there are also purl increases for the wrong side.
Role in fabric shaping.
The typical use for increases and decreases is to taper a flat piece of knitted fabric, e.g., in widening the sleeve of a sweater from the wrist to the biceps. However, increases and decreases can also be used to produce curvature, i.e., to make the knitted fabric cup (positive curvature) or gather (negative curvature); in woven fabrics, this curvature is produced with darts, flares and gussets. A great advantage of knitted fabrics is that they can be shaped nearly invisibly without seams or cutting (as woven fabrics must) with increases, decreases, and/or short rows. 
Lace patterns also make use of increases by immediately combining them with increases. As a result, the stitch count doesn't change but the knitter can create stunning patterns through the eyelets, bars, and slanting lines these stitches create.
References.
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