Double Contact Bayonet
These are common bulbs in RVs and Boats. They have 2 contacts on the bottom, and little pins across from each other on the sides of the base. These bulbs are often referred to Indexed or Non-Indexed, this refers to the pins on the sides of the base.
If it is indexed, the pins are at un-even heights. These are commonly used for navigation lights in larger boats and yachts. Commonly in Aqua Signal Series 40 Navigation Lights
If it is non-Indexed then the pins are at an even height, close to the bottom of the base. These are commonly used for interior bulbs in dome lights, and in navigation lights for smaller boats.
Common Bulb model numbers in this category are #1142, #1157, #90