Unusual suit

Generally, an unusual suit refers to a suit bid which 'does not make sense' and is used to indicate something else. It is mainly used after opener wants to show a strong hand (to prevent responder from passing when holding a minimum hand) and a fit for partner's suit at 1-level. (It is also used if opener has a 6-card major after responder bids 2NT, or after opener shows a 6-card major but in different settings.) The details are as follows:

Bidding Sequence	Unusual suit bid	Responder's next bid
		No slam interest	Slam interest No first round control	Slam interest With first round control
		Bid game directly(4H/4S/5D)	Bid the trump suit at the lowest level	Bid the lowest suit with first round control (!!)
1C > 1H	2D (!!)
1C > 1S	2D (!!)
1D > 1H	2S (!!)
1D > 1S	2H (!!)
1H > 1S	3NT (!!)		4NT (Blackwood)

 
Basically the unusual suit to bid is simply the lowest bid that 'does not make sense'.