The old school definition of "flight" is a collection or grouping of similar objects such as a flight of birds etc. Flights generally represent a grouping of wines that share some characteristics, hence a collection of wine tastings that is more than two in number is called a flight. The term really does just mean, a grouping. 

The grouping can be structured around a theme (a flight of whites) or arbitrary (“pick five wines to sample from this list”). In terms of volume, it can mean samples (typically a 1 oz pour) to full glasses! (Not to dash your dreams, but a full glass in the wine world is a 4 to 5 oz pour – each bottle pours about 4 glasses.)

Wine flights in a tasting room environment.

 Generally, a winery will have a standard list of wines which are opened and poured every day. Many wineries offer more than one wine tasting experience, often at different price points. For instance, a winery with both widely distributed and limited production wines might have two (or more) groupings, or flights. A tasting of the widely distributed wines (usually with a lower price point) might be called the standard wine flight and be priced at say, $10 to $20. A tasting of the more limited production wines might be called a reserve flight, and will typically have a higher price tag. To avoid surprises, do make sure you understand what is being offered and what you’re ordering!