Any small "seed" pattern in Conway's Life that does not stabilize for a very long time. The term is not used to describe objects that are known to grow forever, and was originally defined as a pattern having a population of at most 10 and a Life-span of at least 100. Examples include the acorn, bunnies, pi heptomino, r-pentomino, rabbits, and thunderbird, as well as the unnamed patterns below. The first becomes the r-pentomino after two ticks. The second stabilizes after 608 generations into six blocks, 12 blinkers, and one loaf. The third lasts 1108 ticks, and the fourth lasts 148 ticks, after forming three blocks, a ship, and two gliders.
Source: Life Lexicon by Stephen Silver
and Eric Weissten's Treasure Trove