These days, the trend is toward smaller niches (Targeting teenagers isn't specific enough; targeting male, African-American teenagers with family incomes of $40,000 and up is. Aiming at companies that sell software is too broad; aiming at Northern California-based companies that provide internet software sales and training and have sales of $15 million or more is a better goal.
Concentrating all marketing efforts on a small but specific and well defined segment of the population. Niches do not 'exist' but are 'created' by identifying needs, wants, and requirements that are being addressed poorly or not at all by other firms, and developing and delivering goods or services to satisfy them. As a strategy, niche marketing is aimed at being a big fish in a small pond instead of being a small fish in a big pond. Also called micromarketing.