Great affliction, trial, distress and/or suffering, making it hard to bear physically or emotionally. An experience or set of events that tests one's resiliency, character, endurance, patience, and even Faith.  

Acts 14:22
Confirming the souls of the disciples, and exhorting them to continue in the faith, and that WE MUST, through MUCH TRIBULATION, enter into the Kingdom of God. 

Romans 12:12
Rejoicing in hope; patient in TRIBULATION; continuing instant in prayer ...

Romans 5:3
And not only so, but we glory in TRIBULATIONS also: knowing that TRIBULATION WORKETH PATIENCE.   

1 Peter 4:12
Beloved, think it not strange concerning the FIERY TRIAL which is to TRY you, as though some strange thing happened unto you: But rejoice, inasmuch as ye are partakers of Christ's sufferings; that, when His glory shall be revealed, ye may be glad also with exceeding joy.

Ephesians 3:13
Wherefore I desire that ye faint not at my TRIBULATIONS for you, which is your glory. 

2 Thessalonians 1:4
So that we ourselves glory in you in the churches of God for your patience and faith in all your persecutions and tribulations that ye endure ... 

1 Peter 1:7
That the TRIAL OF YOUR FAITH, being much more precious than of gold that perisheth, though it be tried with fire, might be found unto praise and honour and glory at the appearing of Jesus Christ ...

Hebrews 11:36-38
And others had trial of cruel mockings and scourgings, yea, moreover of bonds and imprisonment: They were stoned, they were sawn asunder, were tempted, were slain with the sword: they wandered about in sheepskins and goatskins; being destitute, afflicted, tormented; (Of whom the world was not worthy:) they wandered in deserts, and in mountains, and in dens and caves of the earth.  

2 Corinthians 11: 24-27
Of the Jews five times received I forty stripes save one Thrice was I beaten with rods, once was I stoned, thrice I suffered shipwreck, a night and a day I have been in the deep; In journeyings often, in perils of waters, in perils of robbers, in perils by mine own countrymen, in perils by the heathen, in perils in the city, in perils in the wilderness, in perils in the sea, in perils among false brethren; In weariness and painfulness, in watchings often, in hunger and thirst, in fastings often, in cold and nakedness. 

Acts 9:15-16
But the Lord said unto him, Go thy way: for he is a chosen vessel unto me, to bear my name before the Gentiles, and kings, and the children of Israel: For I will shew him how great things HE MUST SUFFER for my name's sake.  

Matthew 13:19-23
When any one heareth the Word of the kingdom, and understandeth it not, then cometh the wicked one, and catcheth away that which was sown in his heart. This is he which received seed by the way side. But he that received the seed into stony places, the same is he that heareth the word, and anon with joy receiveth it; Yet hath he not root in himself, but dureth for a while: for when TRIBULATION or persecution ariseth because of the Word, by and by HE IS OFFENDED. He also that received seed among the thorns is he that heareth the word; and the care of this world, and the deceitfulness of riches, choke the word, and he becometh unfruitful. But he that received seed into the good ground is he that heareth the word, and understandeth it; which also beareth fruit, and bringeth forth, some an hundredfold, some sixty, some thirty.  

Deuteronomy 4:29-31
But if from thence thou shalt seek the LORD thy God, thou shalt find him, if thou seek him with all thy heart and with all thy soul. When thou art in TRIBULATION, and all these things are come upon thee, even in the latter days, if thou turn to the LORD thy God, and shalt be obedient unto his voice; (For the LORD thy God is a merciful God;) He will not forsake thee, neither destroy thee, nor forget the covenant of thy fathers which he sware unto them. 

2 Corinthians 1:3-4
Blessed be God, even the Father of our Lord Jesus Christ, the Father of mercies, and the God of all comfort; Who comforteth us in all our TRIBULATION, that we may be able to comfort them which are in any trouble, by the comfort wherewith we ourselves are comforted of God.  

John 16:33
These things I have spoken unto you, that in me ye might have peace. In the world ye shall have TRIBULATION: but be of good cheer; I have overcome the world.