The term start up casual refers to a dress code typically found in small companies that have just launched their business. Startup casual is slightly less formal than business casual, which is more common in larger organizations.
One facet of a company's culture is its dress code. When a company first launches, its employees may be asked to cover a variety of technical areas, and the effort involved oftentimes means long hours. To create a more comfortable atmosphere, a dress code has evolved that is a little less formal than business casual, and it's appropriately named startup casual. Generally, this attire would include slightly more colorful clothes that might also be described as youthful.
For men, this attire would include a dark pair of jeans or khakis. Faded jeans or ripped pants would not be representative of this style of dress. Footwear for men would include loafers or a very clean (relatively new) pair of sneakers. Shirts can range from well-constructed T-shirts through polo shirts, button downs and sweaters. The point of startup casual is to be comfortable; this dress code does not give employees permission to dress nonchalantly.
As was the case with men, women will wear a dark pair of jeans or khakis. Tops would include simple blouses through light sweaters. Footwear for women includes both flats and wedges. Again, the point is to wear neat but casual attire that remains comfortable after a long day's work.