Meaning: a language in visual-spatial modality (signing) used by Deaf people; the opposite medium of vocal-aural modality (speech) used by hearing people.
Signed language is a term equivalent to spoken language. When using one of the terms in either spoken or signed forms, be aware of whichever modality or language you're talking about.
ASL is a language rather than "a sign language" because ASL is a language, no less than spoken language as we don't say English / French / other (speech) language is a "speech language" instead of "language".
Another thing to deconstruct, people often say "I learned sign language", "I know sign language a little". Which language? Sign language. Which sign language? ASL. Okay. We don't say "I'm learning a new speech language." Right? Which speech language? French? Spanish?
Review 101 words. Refresher.