よほど (and its casual variant よっぽど) is an adverb meaning 'very', 'greatly', or 'much more than expected'. It often carries a nuance of 'considerably' or 'to a surprising extent', and is commonly used when comparing a situation to a norm or expectation. For example, if you say 今日はよほど暑い, it implies the heat is remarkable, perhaps more than usual. It can also be used with a verb in the past tense to mean 'almost did something' or 'was very close to doing something' (e.g., よほど言おうと思った). This usage is common in casual speech.

よっぽど is the more colloquial form, often used in conversation, while よほど is neutral and can appear in both speech and writing. Both are interchangeable in most contexts, but よっぽど feels more informal. When used with a negative or with そうだ, it can mean 'very much so' or 'to a great extent'.

Note: よほど is not used to simply say 'very' in a neutral way; it emphasizes a degree that stands out or is beyond the ordinary. So you would not say よほど美味しい for a normal tasty meal; you'd say it for something exceptionally delicious.