Carbon neutral - activity or product that does not add to the net amount of free carbon in the global carbon cycle -
For something to be carbon neutral it has to balance its "carbon budget" -
Fossil fuels are net carbon emitters or "carbon positive"· because they release carbon that was locked away by geological processes back into the system - adding to the net total of free carbon.
Carbon neutral activities balance their overall budget in a reasonable timescale - so using fast growing biomass for fuel would be carbon neutral in combustion - the carbon released would be re-absorbed by its regrowth - It would not necessarily be entirely carbon neutral - if the energy cost of harvesting it, transport fertiliser etc came from fossil fuels it would still have some carbon costs - but it is a "carbon friendly" way of obtaining fuel.
Carbon negative activities lock up carbon - usually be using biomass in long term ways and taking free carbon out of the system - e.g. straw bale buildings, biochar, planting forests etc.