Here are several definitions, and they all say the same thing in different ways.

God has a good plan for you, to attain to God’s glory.  In your current condition, you have not received all that God has for you.  To miss all of this glory is sin.

Sin is the reason for death and suffering.  Had Adam never sinned, there would be no death and no suffering.  The Bible tells us that the last enemy to be defeated will be death. 

Sin and death are the two things that Jesus came to set us free from. 

Rebellion, wanting to be autonomous from God, separates us from God.  When we are separated from God, we cannot be godly--how would it be possible to be godly?  When we are separated from God, we miss out on His glory, His leading, and His power for righteousness.

The lack of love is sin.  This is another way of saying the same thing.

Disobeying the commandment of God is sin.  God's commandment is to love.  Adultery, Fornication, Stealing, Lying, Cheating, Envying, Killing; these are all examples of a lack of love.

A desire to NOT glorify and respect God and NOT hold Him in holy reverence is sin.

A desire to add to and take away from God's Word and to follow one's own mind and will is sin, which leads to spiritual death and is the cause of physical misery, sickness, and death.

The path toward the fulfillment of what God intends to do in our lives is opened by being joined to God.  We can only be joined through Jesus.  We need forgiveness.  We need to bridge the gap.  We cannot possibly do it ourselves.  Even our righteousness is as filthy rags to God.   God had to make the ultimate sacrifice to win us back to Himself.
Sin "may be regarded as the missing of a mark or aim." ~ Archbishop Trench

See also: What Is Sin?

Below are some some literal translations of the words that are used in the Bible for sin:

02398 ajx chata' khaw-taw' 
a primitive root; TWOT-638; v
1) to sin, miss, miss the way, go wrong, incur guilt, forfeit, purify from uncleanness
1a) (Qal)
1a1) to miss
1a2) to sin, miss the goal or path of right and duty
1a3) to incur guilt, incur penalty by sin, forfeit
1b) (Piel)
1b1) to bear loss
1b2) to make a sin-offering
1b3) to purify from sin
1b4) to purify from uncleanness
1c) (Hiphil)
1c1) to miss the mark
1c2) to induce to sin, cause to sin
1c3) to bring into guilt or condemnation or punishment
1d) (Hithpael)
1d1) to miss oneself, lose oneself, wander from the way
02399 ajx chet' khate
from 02398; TWOT-638a; n m
1) sin
1a) sin
1b) guilt for sin
1c) punishment for sin

07686 hgv shagah shaw-gaw' 
a primitive root; TWOT-2325; v
1) to go astray, stray, err
1a) (Qal)
1a1) to err, stray
1a2) to swerve, meander, reel, roll, be intoxicated, err (in drunkenness)
1a3) to go astray (morally)
1a4) to commit sin of ignorance or inadvertence, err (ignorantly)
1b) (Hiphil)
1b1) to lead astray
1b2) to lead astray, mislead (mentally)
1b3) to lead astray (morally)

264 amartanw hamartano ham-ar-tan'-o
perhaps from 1 (as a negative particle) and the base of 3313; TDNT-1:267,44; v
1) to be without a share in
2) to miss the mark
3) to err, be mistaken
4) to miss or wander from the path of uprightness and honour, to do or go wrong
5)  to wander from the law of God, violate God's law, sin

265 amarthma hamartema ham-ar'-tay-mah
from 264; TDNT-1:267,44; n n
1) sin, evil deed
For Synonyms see entry 5879

266 amartia hamartia ham-ar-tee'-ah
from 264; TDNT-1:267,44; n f
1) equivalent to 264
1a) to be without a share in
1b) to miss the mark
1c) to err, be mistaken
1d) to miss or wander from the path of uprightness and honour, to do or go wrong
  1e) to wander from the law of God, violate God's law, sin
2) that which is done wrong, sin, an offence, a violation of the divine law in thought or in act
3) collectively, the complex or aggregate of sins committed either by a single person or by many

0819 hmva 'ashmah ash-maw' 
from 0817; TWOT-180c; n f
1) guiltiness, guilt, offense, sin, wrong-doing
1a) doing wrong, committing a trespass or offense
1b) becoming guilty, guilt
1c) bringing a guilt-offering

02401 hajx chata'ah khat-aw-aw' 
from 02399; TWOT-638d; n f
1) sin, sin offering
1a) sin
1b) sin offering

02403 hajx chatta'ah khat-taw-aw'  or tajx chatta'th khat-tawth' 
from 02398; TWOT-638e; n f
1) sin, sinful
2) sin, sin offering
2a) sin
2b) condition of sin, guilt of sin
2c) punishment for sin
2d) sin-offering
2e) purification from sins of ceremonial uncleanness

05771 Nwe `avon aw-vone'  or Nwwe `avown (#2Ki 7:9; Ps 51:5) aw-vone' 
from 05753; TWOT-1577a; n m
1) perversity, depravity, iniquity, guilt or punishment of iniquity
1a) iniquity
1b) guilt of iniquity, guilt (as great), guilt (of condition)
1c) consequence of or punishment for iniquity

0819 hmva 'ashmah ash-maw' 
from 0817; TWOT-180c; n f
1) guiltiness, guilt, offense, sin, wrong-doing
1a) doing wrong, committing a trespass or offense
1b) becoming guilty, guilt
1c) bringing a guilt-offering