The vanguard of an army is the part of it that goes into battle first. a force of mobile reserve units that could strike quickly and effectively at the vanguard of an invading army. Any of three unmanned U.S. experimental satellites. Vanguard I (1958), the second U.S. satellite placed in orbit around Earth (after Explorer 1), was a tiny 3.25-lb (1.47-kg) sphere with two radio transmitters. Its flight path revealed that Earth is almost imperceptibly pear-shaped, confirming earlier theories. Vanguard II (1959) carried light-sensitive photocells to provide information about Earth's cloud cover, but its tumbling motion rendered the data unreadable. Vanguard III (1959) was used to map Earth's magnetic field
If someone is in the vanguard of something such as a revolution or an area of research, they are involved in the most advanced part of it. You can also refer to the people themselves as the vanguard. Such thinking puts Kodak in the vanguard of a movement reshaping the computer industry the role of the Party as the political vanguard