"A distribution method for software through a network interface." (DAMA International, "The DAMA Dictionary of Data Management", 2011)
"Applications that are licensed to customers for use as a service on demand." (Gina Abudi & Brandon Toropov, "The Complete Idiot's Guide to Best Practices for Small Business", 2011)
"a software deployment model where a provider licenses an application to customers for use over the Internet, without requiring purchase and installation of the licenses." (Bill Holtsnider & Brian D Jaffe, "IT Manager's Handbook" 3rd Ed., 2012)
"The delivery of computer applications over the Internet." (Marcia Kaufman et al, "Big Data For Dummies", 2013)
"A delivery mechanism in which an application and all of the associated resources are provided to organizations by a vendor, typically through a web browser. Commonly abbreviated as SaaS." (Manish Agrawal, "Information Security and IT Risk Management", 2014)
"Abbreviation for software as a service. It is the capability provided to the consumer to use the provider’s applications running on a cloud infrastructure. The applications are accessible from various client devices through a thin client interface such as a web browser (e.g., web-based email). The consumer does not manage or control the underlying cloud infrastructure, including network, servers, operating systems, storage, or even individual application capabilities, with the possible exception of limited user-specific application configuration settings." (James R Kalyvas & Michael R Overly, "Big Data: A Businessand Legal Guide", 2015)
"Software as a Service is the delivery of computer applications over the Internet on a per user per month charge basis." (Judith S Hurwitz, "Cognitive Computing and Big Data Analytics", 2015)
"A model of software deployment or service where customers use applications on demand." (Mike Harwood, "Internet Security: How to Defend Against Attackers on the Web" 2nd Ed., 2015)
"An approach to software licensing and delivery in which software is hosted remotely in the cloud and accessed via an Internet browser." (Jonathan Ferrar et al, "The Power of People: Learn How Successful Organizations Use Workforce Analytics To Improve Business Performance", 2017)
"Cloud application services in which applications are delivered over the Internet by the software provider, typically for a monthly fixed fee. The applications are not installed, nor do they run on the client’s computers; instead, they are accessed by a Web browser. Two important characteristics of SaaS are as follows: Network and Web-based access to commercial software computing services in which the processing is done on a third party server, rather than at each customer’s location. A tenant-based pricing model for hardware, software, administration, and consulting services." (John H Higgins & Bryan L Smith, "10 Steps to a Digital Practice in the Cloud" 2nd Ed., 2017)
"Software as a service refers to the delivery of software-based business tools via the Internet as an alternative to traditional on-premise installations." (Informatica) [source]
"Software-as-a-service (SaaS) is a model of software distribution where customers pay a monthly subscription or licensing fee and a third-party, typically the software vendor, makes the application available over the internet. SaaS is one of the primary commercial applications of cloud computing, along with infrastructure-as-a-service (Iaas) and platform-as-a-service (PaaS)." (Sumo Logic) [source]