What keyword is essential when defining methods in an Apex class to expose it as a web service?

The keyword that is essential when defining methods in an Apex class to expose it as a web service is "webservice." This keyword indicates that the method can be called by external clients, allowing it to be accessed over the web using protocols like SOAP or REST. When you use the "webservice" keyword, it marks that particular method as part of the service interface, making the method available for invocation from outside Salesforce. Without this keyword, the method would not be recognized as a web service operation by external systems, thus preventing integration.

The other keywords may have their specific purposes, but they do not serve to expose methods as web services in Apex. For example, using "public" allows a method to be accessible from other classes but doesn't make it a web service. "Static" indicates that the method can be called without instantiating an object of the class, but again, this does not define the method as a web service. "Private" restricts access to the method within the same class, which is contrary to the intention of exposing functionality for external access. Hence, "webservice" is vital for enabling the interaction with external applications.