There is no general agreement on the definition of an "oil plume". Some scientists, the media and the public understand the term to mean a body of oil that is on, or under the surface of the water or is spreading on or into the ground. This requires the presence of a body of oil or its components that is identifiable or measurable.
In water, the issue is of identifying bodies of hydrocarbons that have been dissolved or dispersed. The dispersant, itself, is considered to be operating as part of the body, or "plume".
These bodies are not necessarily cohesive and visible blobs, sheens or bubbles of oil and/or dispersant. The bodies may even contain particles that are broken down into microscopic particles that are invisible to the naked eye. Such plumes require advanced methods of chemical detection and data modelling to estimate the size and location of the mass. Until microscopic plumes are identifiable, their existence can be argued about and even denied.
Also, there are plumes that have settled and which ride on various levels in water that is at least a mile deep. As the oil travels through the straits of Florida, it may undergo any form of transport, sinking and breaking up through weathering. Many of these plumes will either not be approachable or they will not be visible enough for conventional methods of detection and monitoring.
In addition, there are concentrations of hydrocarbons that are either toxic or nontoxic. Some scientists and litigants are only concerned with the toxic component, while others are concerned about the presence of any hydrocarbons. One might call something a plume while another will challenge using the term "plume" for describing the presence of crude oil components, based simply on their nature.
There are also atmospheric crude oil and gas plumes that are formed when various gases vent off from the oil or vent directly into the atmosphere. The gases can be toxic, flammable, or have untold effects on the atmosphere.
The bottom line is the way in which the mass of crude oil, with its pockets of pure gases, is dispersed, either by weathering, by application of man made dispersants, by water movement and currents, by burning, and by other natural and anthropogenic causes.
The presence of any of the components of out of place crude oil can be called a "plume". The problem is that the sheer area and volume of the water or land, the sheer volume of oil that is being released, along with the complexities of weathering, currents or dispersing make for a very difficult identification of anything but visible and accessible masses of hydrocarbons that are in the water or land.
To further complicate matters with the BP oil spill of 2010, the requirements and ploys that are involved in civil and criminal litigation combine with the fact that much of science is in a study and exploratory mode. This will make it difficult, if not impossible to prove that even the huge masses of visible oil present a hazard, cause harm, or will even exist.
The most obvious plumes may eventually be vacuumed up and enough of the mass removed from visibility. Or, the plumes may sink or soak into atmospheric, underground, and underwater soil and water tables where they will be impossible to locate, measure and monitor.
Many plumes will exist as only invisible and microscopic components are left behind. Or, they may become so dispersed as to be impossible to detect. Today's scientific understandings may not be enough to prove harm and damage or to gain compensation.
In summary, the bad news is that the words "oil plume" can have many meanings and many definitions, with no official definition at this time. It is, therefore, important to know the context, specific definition and meaning that each speaker or media source intends when using the term.