Does "stream enterer" simply mean "devout Buddhist"? I have read about technicalities of stream entry, but just wanted to ask, to see if I understand the "big picture": If we step outside and try to describe it in plain English, is a "stream enterer" simply a "devout Buddhist"?

Let me offer an analogy with a more typical use of this word: a devout Catholic is a person who practices Catholicism not because of his upbringing/pressure from the society/habits of following the rituals/suspicion that God may exist, but because he knows that this is the right thing to do, and he knows that God exists (perhaps because of some experience that completely conviced him and left no doubt). So, is a stream enterer, analogously, somebody who knows that suffering is the problem, and knows that the Buddhist path is the solution (etc.)?

I'm only asking about the concept, and I don't want to get into discussion whether things that a devout of a particular religion knows are may be true or not.
No, a stream enterer is much more than that. A stream enterer has experienced Nibbana first hand and is changed in a permanent way because of it. Stream entry totally destroyes the first three fetters of self view, doubt about the validity of the teachings, and clinging to rites and rituals.

A person who knows the teachings of the Buddha are true and sincerely follows them to seek enlightenment but hasn't yet attained stream entry would be called a kaliyana putthujana, or good worldling.

(Added later)

Some other terms that might be what you are looking for are the terms Faith-follower and Dhamma-follower, which refer to individuals who have come to have come to an acceptance of the Dhamma either primarily through the quality of faith or of wisdom, but have not yet been able to directly perceive the truth of the Dhamma for themselves. For example, the Vinnana Sutta says:


  At Savatthi. "Monks, eye-consciousness is inconstant, changeable,
  alterable. Ear-consciousness... Nose-consciousness...
  Tongue-consciousness... Body-consciousness... Intellect-consciousness
  is inconstant, changeable, alterable.
  
  "One who has conviction &amp; belief that these phenomena are this way is
  called a faith-follower: one who has entered the orderliness of
  rightness, entered the plane of people of integrity, transcended the
  plane of the run-of-the-mill. He is incapable of doing any deed by
  which he might be reborn in hell, in the animal womb, or in the realm
  of hungry shades. He is incapable of passing away until he has
  realized the fruit of stream-entry.
  
  "One who, after pondering with a modicum of discernment, has accepted
  that these phenomena are this way is called a Dhamma-follower: one who
  has entered the orderliness of rightness, entered the plane of people
  of integrity, transcended the plane of the run-of-the-mill. He is
  incapable of doing any deed by which he might be reborn in hell, in
  the animal womb, or in the realm of hungry shades. He is incapable of
  passing away until he has realized the fruit of stream-entry.
  
  "One who knows and sees that these phenomena are this way is called a
  stream-enterer, steadfast, never again destined for states of woe,
  headed for self-awakening."
  http://www.accesstoinsight.org/tipitaka/sn/sn25/sn25.003.than.html