Is Mindspark.A a virus or Malware

Or, 3. Microsoft has different criteria.

According to this removal guide, this detection naming is specific to Malwarebytes:

http://malwaretips.com/blogs/pup-optional-mindspark-removal/

And it is "adware"

"PUP.Optional.MindSpark it’s technically not a virus, but it does exhibit plenty of malicious traits, such as rootkit capabilities to hook deep into the operating system, browser hijacking, and in general just interfering with the user experience. The industry generally refers to it as a “PUP,” or potentially unwanted program.

The PUP.Optional.MindSpark infection is used to boost advertising revenue, as in the use of blackhat SEO, to inflate a site’s page ranking in search results.

PUP.Optional.MindSpark got on your computer after you have installed a freeware software (video recording/streaming, download-managers or PDF creators) that had bundled into their installation this browser hijacker. This Potentially Unwanted Propgram is also bundled within the custom installer on many download sites (examples: CNET, Brothersoft or Softonic), so if you have downloaded a software from these websites, chances are that PUP.Optional.MindSpark was installed during the software setup process.

PUP.Optional.MindSpark is an ad-supported (users may see additional banner, search, pop-up, pop-under, interstitial and in-text link advertisements) cross web browser plugin for Internet Explorer (BHO) and Firefox/Chrome (plugin) and distributed through various monetization platforms during installation. The browser extension includes various features that will modify the default or custom settings of the browser including the home page, search settings and in some cases will modify Internet Explorer’s load time threshold, place a lock file within Firefox to prevent competing software from changing its settings as well as disable the browser’s Content Security Policy in order to allow for cross site scripting of the plugin."

MalwareBytes has detected numerous registration keys for Mindspark.A and quarantined them.

Is Mindspark.A a virus or Malware; how significant are it's conseequences, how to remove completely and does it keep returning?

It appears that Microsoft's Security Scanner built into OS 8 and optional to OS 7 just leaves this item alone. MalwareBytes treats it as a threat. Two thoughts come to mind. One Microsoft is a poor program or two Microsoft is complaisant with the people writing the programs.

Mindspark is a malicious toolbar which is able to causes browser problems. Mindspark.A can be a variant of Mindspark.  It can hijack homepage, redirect users from their search results, and is able to inject other infections to your computer. Here is the guide to disable a toolbar (take IE as example): 

launch Internet Explorer >> go to Tools >> Manage add-ons >> Toolbars and Extensions section >> select Mindspark.A and press the button Disable >> restart Internet Explorer. You can consult online computer expertsif you have any questions.