I use both interchangeably. What's the difference?
When used in the context of giving oneself wholly to a cause/course of action etc., there is no real difference.
One is used more than the other in common phrases, but that's about it. For example you would usually hear:
He was a dedicated police officer.
He was a committed police officer.
Though both are equally valid. Dedicated in this context gives the impression of more zealous, heroic commitment.
Commitment can also imply obligation:
"Where's John?" "He's got work commitments."
Meaning he is obliged to be elsewhere because he has committed himself to work.
I'd say there is little effective difference, and I'd disagree with Elendil that
dedicated is somehow more heroic, however the ever-so-slight differences are:
- Committed suggests that one is bound or obligated; perhaps one has made a pledge
- Dedicated suggest that one is devoted, but without there being a formal commitment.
In essence, once suggests a possibly-voluntary obligation and the other suggested a wholly-voluntary devotion, but in most everyday use, the terms are synonymous.
Committed means (as reported from the New Oxford American Dictionary) feeling dedication and loyalty to a cause, activity, or job; wholeheartedly dedicated.
Dedicated means devoted to a task or purpose; having single-minded loyalty or integrity.
There is a slight difference between the words; a difference is that dedicated can also be refer to a thing, and in that case it means exclusively allocated to or intended for a particular service or purpose.
Dedication and commitment are synonyms to a certain degree. However, dedication is almost always used in relation to a task, responsibility or something to be done, while commitment embodies devotion or allegiance to anything—task, cause, person, etc. Thus, commitment could be regarded as a hypernym of dedication.
They are very close synonyms, but not interchangeable.
Commitment most often refers to the promise itself.
Dedication most often refers to the making of the promise (an event).
This distinction is amplified when plural.
In the context of possession, they're more interchangeable:
His dedication was apparent to all who knew him.
But *Did you see Joe's dedication?" is ambiguous. It could mean *Did you hear Joe make a promise?" or "Did you see evidence of him following through on his promise?"
Usage of the verb forms also differs. Both are transitive, but only "commit" can be used intransitively with an implied reflexive direct object:
I commit (myself) to serving my community.
The implied direct object cannot be omitted with "dedicate", whereas it would be slightly awkward to include it with "commit".
I'm sorry that I can't give a simple rule for which to use, it just comes naturally to the native speaker.
The only way to understand the difference between synonyms such as commitment and dedication is to find the origin of the words and their current usage.
commit means to consign, to pledge, to engage
dedicate means to set apart, to devote, to offer something in testimony of respect
dedication holds more associations to honor, higher spiritual or long lasting values than commitment
In that sense the subtle difference is probably best understood if we consider the longevity of the task or purpose people are committed or dedicated to.
dedicate shares some of its association scope with devote, commit has no strong relations to devote.
commitment is used (Google) 162.000.000 times, commitments 40.000.000 times, ratio 4:1 dedication is used 55.000.000, but only 3.000.000 times the plural dedications is used, ratio 18:1
This suggests that dedication is used as a noun with abstract meaning far more frequently than commitment and this suggests that dedication is somewhat deeper than commitment. Commitment is more present.
The difference is subtle, of course, but these are suggestions in the right direction.
A strong example that shows how subtle the difference could be:
I commit my life to God. the life is taken as a chain of good and bad events where I want to allow a divine intervention each time
I dedicate my life to God. the life is taken as me and any action or work of mine which I will try to offer as a gift to God
Quiting smoking could be a commitment, but not a dedication. Going to war could be both.