<jargon> 1. Extremely small. "A marginal increase in core can decrease GC time drastically." In everyday terms, this means that it is a lot easier to clean off your desk if you have a spare place to put some of the junk while you sort through it.
2. Of extremely small merit. "This proposed new feature seems rather marginal to me."
3. Of extremely small probability of winning. "The power supply was rather marginal anyway; no wonder it fried."