I am not able to maintain the erection inside vagina only. while i masturbate, i am able to maintain erection. is this erectile dysfunction?

If it doesn't work when you need it to its ED. See a urologist who specializes in the issue to plan an approach to therapy.

No. If you get an erection the mechanism for it is intact- not being able to sustain it in the vagina seems more due to psychological cause. If you can prevent that by persistence and will power - it is the best. Otherwise pl see a sex thearpist