What is the difference between retrospect & hindsight?

Can they be used interchangeably, or is there a difference?

Retrospect is simply looking back at the past.

Hindsight is a perspective on the past in which understanding has been gained which was unavailable to the observer at the time in question.

Retrospection can confer hindsight, but it is not part of the definition. A sentence containing the phrase "in retrospect" often describes a new appraisal of the past time or event but I would not say it is essential.

Some dictionary definitions to clarify the difference:

Retrospect

• noun: 1. regard or reference to precedent or authority, or to previous conditions. 2. a survey of past time or events. (Oxford American Dictionary) a view or contemplation of something past; also, that which is contemplated. (Funk & Wagnall)
• verb (intransitive): to refer back, to reflect (Merriam-Webster) to look back, to consider the past (Funk & Wagnall)
• verb (transitive): to go back over in thought (Merriam-Webster) To consider or revert back to the past of (Funk & Wagnall)

Hindsight

• noun: the knowledge and understanding that you have about an event only after it has happened (Merriam-Webster) wisdom after the event (Oxford American Dictionary) knowledge based on experience (Funk & Wagnall)

The definitions of hindsight stress the new (changed/superior/matured) perspective gained after the event. That is the essential nature of hindsight. In contrast, the definitions of retrospect refer simply to the act of looking back. Consideration is mentioned in places but reconsideration is not. Any fresh evaluation would depend upon the intention and perceptiveness of the actor - it is not automatically implicit.

Note that the phrase "In retrospect" does strongly imply hindsight. Its inclusion in a sentence indicates that things would be/seem different if the past were not being considered. I would argue that the need to add "In" - to create contrast between the situation with reference to the past as opposed to that without such reference - only emphasises the fact that "retrospect" by itself does not imply hindsight.

In answer to your question, to use retrospect in place of hindsight will typically lose important meaning, while replacing retrospect with hindsight will often add unintended semantics.

Obviously both convey they concept of looking backwards, however it's meaningful that one contains a verb and the other doesn't.

First let's analyse by inverting them. The inverse of retrospect would be 'look forward', while the inverse of hindsight is 'foresight'. Immediately we begin to sense a greater gap, we might use retrospect and hindsight very similarly, but we use 'look forward' and 'foresight' very differently.

Next let's dissect them, to see if that difference is consistent.

Retrospect contains a Latin root meaning 'look at'. So it may appropriately be said that "in retrospect" is process focussed, and that results may or may not come from it.

Hindsight references either an ability or a state. Sight is being able to see, or the having of knowledge. So hindsight focusses more on the result than the process.

{I was surprised to find that hindsight may actually have entered English as an 1800's proverbial contrast to foresight, it "felt" like a much older word.}

So the end analysis is that while common usage is interchangeable the words have a depth of meaning that indicates when to use either one.