Username + password = one word
Consider "Enter your login credentials".
Credentials
Usually, credentials. evidence of authority, status, rights, entitlement to privileges, or the like, usually in written form: "Only those with the proper credentials are admitted."
Anything that provides the basis for confidence, belief, credit, etc.
If you really want one word:
- Enter your authentication.
- Enter your credentials.
- Enter your Information.
If you're trying for a single word, "authentication" is likely to be the best, but it's going to depend on the audience.
I wouldn't try to do it as a single word. If you don't want to have labels before each field, I'd write:
Enter your Google username and password
[username]
[password]
Maybe this question would be better posted at ux.stackexchange.com.
Particularly if the authentication method may be anything other than username and password (such as including a one-time code), I recommend:
Enter your XXX login information
(where XXX may be Google, Facebook, etc)