Are these interchangeable terms?
Is there a way by which even non prophets can acquire information/communication from the unseen?
In Arabic language both terms (al-Wahy الوحي and al-Ilhaam الإلهام) have a slight difference and can also be synonyms according the context.
Wahy الوحي: inspiration, revelation etc.
فالوحي لغة هو: الإشارة، والكتابة، والرسالة، والإلهام، و الكلام الخفي وكل ما ألقيته إلى غيرك .
In lissan al-'Arab لسان العرب you may find ilham counted among the synonyms of wahy which are: a sign, a letter, a message, an intuition (inspiration), a hidden speech/communication, one can even say it is any thing you say to an other person.
يقال: وحيت إليه الكلام وأوحيت
It is said: وحيت إليه الكلام وأوحيت wahaytu ilyahi al-Kalaama wa awhayto
meaning: "I've talked to him using words or via a statement".
But literally it says: "I've inspired him or revealed him (with) some words or a statement".
The meanings according to shari'a or the technical terms however make a clear difference:
Wahy means: the order from Allah which was given to the Prophets
التعليم الصادر من الله الوارد إلى الأنبياء،
so only Messengers or Prophets can receive wahy (revelation) according this definition.
There are two ways a revelation can be received:
تعليم بواسطة ملك، وتعليم مباشر بلا واسطة ملك،
A teaching or instruction via an Angel or directly without the intermediation of any Angel. Both can be in wakefulness or while sleeping by a good vision (see for example the long hadith of 'Aisha) or in a narration on the authority of ibn 'Abbas:
Nothing remains of the glad tidings of apostlehood (or prophethood), except good visions which a Muslim sees or someone is made to see for him. ...
(Sahih Muslim, but it also appears with a slightly different wording or narrator chain in other hadith collections such as sahih al-Bukhari, al-Muwatta' and sunan an-Nasa'i)
Note that the instruction via an intermediate has three known ways: - That the Prophet sees the Angel in his real form or appearance (rare) - That the Prophet sees the Angel in a human appearance - That the Prophet fail to see the Angel, but hears some sounds etc.
There are different definitions, what ilham might mean:
One of them is doing or saying (or being guided to do or say) the right thing without being a Prophet.
Ilham is based on the hadith saying:
"Amongst the people preceding you there used to be 'Muhaddithun' (i.e. persons who can guess things that come true later on, as if those persons have been inspired by a divine power), and if there are any such persons amongst my followers, it is `Umar bin Al-Khattab." (sahih al-Bukhari and sahih Muslim) with a different wording)