Market share is:

Below are excerpts from two definitions, with links to their sources.

"Market share describes the relationship between a company's success and the success of its competitors. It is the percentage of an entire industry's sales earned by a particular company during a discrete period of time."

"Market share is the percentage of the total market (or industry) sales made by one firm. ... Share can be reflected as either percentage of sales dollars, percentage of units sold, or percentage of customers. Percentage of sales dollars is the most common reference."

Problems when defining a market:

Here are some of the main characteristics or things that can be used or considered when defining a market or industry:

• NAICS codes, or other industry codes (more about industry codes)
• Revenue or sales, i.e. company / brand size (Microsoft versus a neighborhood web design shop)
• Target customers (Microsoft has a different customer base than a neighborhood web design shop; but both are in the software business!)
• Who compiles the data?!  E.g. a government agency, trade association, research firm, etc.

Below are quick & easy, online sources for market share data ... two publications in the database Business Insights - Essentials, all of which reproduce data from other sources, mostly trade magazine articles. The links will take you to all entries in each publication:  use the Search Within boxes (upper left corner) to add your company name, product or brand to the search.

Online, but a little harder:

Passport has good coverage for consumer and packaged goods primarily (nothing for industrials or manufacturing), but can provide very detailed data on market share.  Market share by two different measures even:  by sales value and by sales volume. 

➤ Use the Categories & Topics Full Tree search menu!  The database contains market data on roughly 80 countries:  the Full Tree Search menu allows you to limit your results most effectively.

IBISWorld provides coverage primarily for the US.  If a keyword search fails (whether company name or product/service), be sure to browse through the NAICS code menu, as that can be a more reliable way to find reports.  Once in a report, look at both the Competitive Landscape and Major Companies tabs.

If the sources above all fail, then you'll need to check in some or all of the following.  These will all entail a greater commitment of time, and require more sophisticated searching and/or more skimming through results.  That's just how it is ...