How semelfactive aspect of a verb that represents a single occasion of an event like knock,hit etc..is perfective and moment defined. whereas,iterative aspect is event that is repeated on single occasion. Is both the aspects similar?Any Specific examples to understand this aspect of verbs indetail.
They are quite similar, and you've isolated the main difference: semelfactive is once, iterative is many times.
Although it is translated "aspect" here, it may not be a verbal property at all in some languages. It may be clearer to think of it is a form of lexical aspect, or Aktionsart (after Smith 1997), with the three (Vendlerian) properties: dynamic, atelic, instantaneous/punctual.
Applied to some actual languages then:
According to Forsyth (1970), iterative and semelfactive used to be considered separate sub-aspects, but then they were set as a perpendicular axis of "grammatical: procedural meaning" to the "true" aspects in Russian of perfective vs imperfective.
- крича́ть is an imperfective verb meaning "to shout"
- кри́кнуть is a perfective verb meaning "to shout" with a semelfactive Aktionsart, indicating one time when this happened
- закрича́ть is a perfective verb derived with an inchoative Aktionsart, "to begin to shout"
- покрича́ть is a perfective verb with a durative Aktionsart, "to shout for a while".
- кри́кивать is an imperfective verb meaning "to shout" with an iterative Aktionsart, indicating this happening several times.
There is a whole group of semelfactive perfectives, the -ну- semelfactives in Russian. There is a whole different group of iterative imperfectives, ending in -вать in Russian.
The Sinitic branch has no traditional morphology to speak of, and it's still under intense exploration. But there is a clear difference between a true one-time semelfactive and an iterative, which in Chinese linguistics is seen as a repeated semelfactive (Yang 1985).
在 zài is a coverb that literally means "to be at/in" but often functions as a progressive aspect marker in the pre-verbal position. With a semelfactive verb like 咳嗽 késòu, it causes it to be interpreted as a repeated action "He is coughing", which is then treated as an activity verb with a form of duration (Peck 2013). The two lexical aspects are syntactically similar, although semantically still distinct.
著 zhe the post-verbal durative particle presents an interesting case. Again, this converts semelfactives into activity-like verbs. But there are cases where a difference exists: this "iterative" semelfactive vs a true durative activity/accomplishment is found in example 21 of Xiao & McEnery (2004).
他 母親 搖著 頭 對 記者 嘆息。 Tā mǔqīn yáozhe tóu duì jìzhě tànxí. He mother shake-DUR head facing reporter sigh 他 母親 低著 頭 對 記者 嘆息。 Tā mǔqīn dīzhe tóu duì jìzhě tànxí. He mother lower-DUR head facing reporter sigh
Here the semelfactive-derived 搖著頭 (shaking one's head) contrasts with the true activity 低著頭 (lower[ed] one's head); the iterative (semelfactive-derived) aspect means that in the observation period, it has happened several times, whilst the true durative (either activity or accomplishment) means that it has remained constant through the observation period.