"Harmful interference" is another term from the jargon of radio regulation. Interference, broadly speaking, is anything that tends to disrupt communication, regardless of source. Not all interference is "harmful", though. Merely being "unwanted" or "annoying" is not enough to rise to the level of "harmful"; the disruption has to be significant or repeated, or must completely prevent all communication, before it rises to the level of harmful. Finally, "harmful interference" never originates from a natural source, so static from lightning storms, no matter how annoying or disruptive, doesn't count as harmful interference.
T1A04: Which of the following meets the FCC definition of harmful interference?
- Radio transmissions that annoy users of a repeater
- Unwanted radio transmissions that cause costly harm to radio station apparatus
- That which seriously degrades, obstructs, or repeatedly interrupts a radio communication service operating in accordance with the Radio Regulations
- Static from lightning storms
The correct answer is C–That which seriously degrades, obstructs, or repeatedly interrupts a radio communication service operating in accordance with the Radio Regulations.
All radio licensees, including amateurs, are prohibited from causing harmful interference with any other station. Stations who are suffering from harmful interference are entitled to relief from that interference, which means that the station causing the harmful interference can be made to stop, by force if necessary, by the governing administration that has authority over that station.
Harmful interference doesn't have to be intentional, although it often is. Jamming (that is, transmitting on top of other transmissions in order to block them) is an especially pernicious form of harmful interference. But that's not the only form of harmful interference; far more common are things like spurious emissions caused by malfunctions or maladjusted transmitters, leakage from cable television systems, and unwanted noise from defective household electronics like computers or televisions. All of these, if severe enough, count as harmful interference that can result in enforcement actions from the FCC.
In practice the FCC's effort to abate harmful interference is proportional to what the licensee suffering the interference pays in licensing fees for their license; since amateurs pay nothing for their licenses the FCC doesn't work terribly hard to resolve interference complaints. Basically that means we have to investigate the circumstances ourselves and wrap the whole thing up nice and pretty so all they have to do is some quick work to verify the complaint before they'll act. Fortunately the ARRL is actually pretty good at this; one of the few things they do do well.
This particular question is one that was improved in this version of the question pool; in the 2006 question pool the question misstated the definition of "harmful interference", leading me to complain about it when I blogged about it about a year ago.