Sheikh al-Albani many times says/describes Imam at-Tirmidhi's narrations as ghareeb (ling. strange). So What does Hadith Gharib (حديث غريب) mean? Does it mean that a hadith is weak?
A gharib hadith is a kind of categorization of hadith based on its chain. It is a kind of ahad hadith, which means that it's not narrated through an abundance of narrators at each level of the chain. There are three kinds of ahad hadith: mash-hoor, `aziz, and gharib. A gharib hadith is one in which there is at least one level (or tier) which only has one narrator in it. An example of a gharib hadith is the famous hadith on intention (niyyah):
Narrated 'Umar bin Al-Khattab:
I heard Allah's Messenger (saws) saying, "The reward of deeds depends upon the intentions and every person will get the reward according to what he has intended. So whoever emigrated for worldly benefits or for a woman to marry, his emigration was for what he emigrated for."
This hadith was narrated from one Companion (`Umar (ra)) to one Successor (`Alqamah), and then at each further level there are 2 or more people.
Note that this kind of categorization is not based on the text of the hadith at all, so it is not a verdict on the "strangeness" of a text or lack thereof.
Since this kind of categorization is only based on the number of people at each level of the chain (and not on their memory or trustworthiness), a gharib hadith can be sahih (authentic), hasan (acceptable), da`if (weak), or even other categories of authenticity.
Source: Usul al-Hadith class