The field got a name when it solidified into a field, just like Natural Language Processing did, or Machine Vision.
I'd say a claim of machine learning being "solidified into a field" is somewhere between false and meaningless.
Just look at neural networks. These may be the most successful machine learning or AI technique ever but they're also the technique most dependent on ad-hoc tweaks and tuning of anything so-far. Which seems rather the oppose of solidified.
The Machine Learning journal (which later became the Journal of Machine Learning) was started in 1986 [0], which is 25+ years from when it was first coined [1].
According to Google Trends [2], both computer vision and artificial neural networks, as topics, were more popular than machine learning until 2010. Afterwards, both subfields drop to their historic lows of search interest while interest in machine learning multiples year over year. It's hard to believe that such rapid growth didn't involve a conflation of taxonomy.
If I remember correctly, google trends looks at the frequency of use of n-grams in a google search corpus. I wouldn't expect this to reflect research activity, as such.
The fact that the journal of machine learning is older than the term also doesn't say much. My disagreement with the previous poster seems to be about whether machine learning is a field of AI or not. Well- if it has a journal (and a conference) (or the other way around), then it's a field of study.
I'd say a claim of machine learning being "solidified into a field" is somewhere between false and meaningless.
Just look at neural networks. These may be the most successful machine learning or AI technique ever but they're also the technique most dependent on ad-hoc tweaks and tuning of anything so-far. Which seems rather the oppose of solidified.