This is the Great Planes ElectriFly 2mm male bullet to 3.5mm female bullet adapter.
Features:

• Connects a motor with 3.5mm Male Bullet Connectors to an Electronic Speed Control with 2mm Female Bullet Connectors

Specifications:Connector #1:2mm Male BulletConnector #2:3.5mm Female BulletWire:NoneWeight:0.03oz (0.95g)