There may be temporary, occasional, partial, imagined, etc. erectile failure, not just failure, period. Oftentimes it comes down to expectation of what is normal. Some have realistic, others unrealistic expectations. Someone who doesn't know this may consider himself intermittently impotent. Someone who was used to having sex every day may come in asking for treatment of impotence, because now that he is older, it's only happening twice a week.
Many men come to the doctor complaining of loss of interest, rather than loss of ability.
Erectile dysfunction refers to the inability to attain a full and functional erection, or inability to attain an erection at all. Viagra addresses only this one issue, that of improving the ability to have a full erection. Before writing out a perscription, most doctors will suggest an evaluation. First of all we look for what may be interfering with sexual functioning. Medicines can lead to loss of libido, or interfere with erection or ejaculation in men, and delay or prevent orgasm in women.