Hi Believer, Shingle is a mass of stones on the shore of a sea or river I think the definite article 'the' is used in your sentence because the dictionary is making a general statement about all things of a particular type (from CCEU), which, in this case, is the shore of a sea or river. Best wishes, PBF
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.