Parasitic advertising may have different meanings according to the context. Meaning in "traditional" advertising differs from meaning in digital display advertising context.
The "traditional" meaning of parasitic advertising refers to the fact of legally taking advantage of an event for advertising purposes without being an official sponsor or partner. Parasitic advertising may also refers to the fact of taking advantage of a competitor’s campaign or event.
For more details about parasitic advertising around an event, see indirect ambush marketing.
In digital advertising context, parasitic advertising is often illegal or non ethic practices based upon the use of adwares.
Parasitic advertising is a part of parasitic marketing.
An example of parasitic or opportunistic advertising: