I need to know whether it is correct to call any series of words an “expression.” Examples:
"The study of natural phenomena has continued for centuries, with significant discoveries being made in recent years."
"The speed of light is constant in a vacuum, but it is slower when light passes through transparent substances."
"There are various ways to generate electricity, including the use of hydroelectric stations and solar cells."
Is it correct to call the highlighted words in each of the above sentences an expression?
Some people say that the word “expression” refers to a common or idiomatic phrase. But I have been freely using the word “expression” with reference to any group of words which follow one after another, such as in the above examples which do not contain any idiomatic phrases but simply state facts.
[Added on 9/11/2018:]
There was a discussion on one forum where some people said that the word “expression” does not refer to any sequence of words but to a particular phrase which is commonly used:
It seems that although some people may think that the word “expression” refers to a commonly used phrase, such as an idiomatic statement, it can be used in linguistic contexts with reference to any series of words which have a grammatical meaning. If this is correct, can the following highlighted series of words be called “expressions”?
“‘As the lightning,” he says, shall he come; not concealed in any corner, but shining everywhere.’”
“We must always be doing this so that if our Lord should ever suddenly come to take us, he may find us carrying this out and then assign a reward far greater than that promised earlier to the one properly carrying out his instructions.”