Here's how I think about a platform — you work in a platform. A platform is designed for a specific business function like marketing, sales, HR, finance, etc. Platforms connect to other platforms, so integration is key.
The important part of the definition is that a platform is something you use every day, and plays a critical part in the success of your role. For example, the Acquia marketing team lives inside of Marketo, setting up programs, writing emails, scoring leads and analyzing results. Marketo is the first application we open in the morning and the last we close at the end of the day. It plays a pivotal role in making sure we're passing the best leads to sales at the right time, while nurturing our earlier stage prospects.