Meanings and pronunciation of 截
1. What are all meanings of 截?

It has the following meanings according to Xinhua Dictionary online:


割断，弄断：截断。截开。截取。截面。(to cut off, to truncate)
量词，段：一截儿木头。 (measure word: a section, a chunk)
阻拦：截住他。截留。截击。截获。(to intercept, to obstruct)
到一定期限停止：截止。 (cut-off point, deadline)


2. (What's the meaning) as in 截拳道?

In 截拳道, 截 means 'to intercept'. The spirit of 截拳道 is to 'intercept' the opponent's intention/omens of attack, and counter the attack accordingly. There is more information on Wikipedia: 


  JKD teaches that the best defence is a strong offence, hence the principle of an "intercepting fist". For a person to attack another hand-to-hand, the attacker must approach the target. This provides an opportunity for the attacked person to "intercept" the attacking movement. The principle of interception may be applied to more than intercepting physical attacks. Non-verbal cues (subtle movements that an opponent may be unaware of) may be perceived or "intercepted", and thus be used to one's advantage.


3. Is it pronounced "Jeet" or "Jee"?

截 is pronunced as jie2 in Mandarin and zit6 in Cantonese. 

Jeet/Jee are non-standard transliterations from Cantonese to English.

4. I'm interested in the word "jeet" and how it may be used.

If you're talking about the English word, I don't think there are widely-known usages other than in 'Jeet Kune Do'.