Overview:
This question is related to the concept of parse trees in grammar. A parse tree represents the structure of a sentence in terms of its grammatical components. The question asks about a specific type of grammar that can produce more than one parse tree for a sentence.

Ambiguous Grammar:
An ambiguous grammar is a type of grammar where a single sentence can have more than one possible parse tree. In other words, there is more than one way to interpret the sentence structure based on the grammar rules. This can lead to different meanings or interpretations of the sentence.

Example:
Let's consider a simple example to illustrate the concept of an ambiguous grammar. Suppose we have the following grammar rules:

1. S -> NP VP
2. NP -> "the" N
3. NP -> "the" N PP
4. VP -> V NP
5. PP -> P NP

Now, let's consider the sentence "the cat chased the mouse with a stick." This sentence can have two different interpretations based on the grammar rules:

Parse Tree 1:
```
S
/ \
NP VP
| |
"the" V
| |
N NP
| |
"cat" |
NP
/ \
"the" N
| |
"mouse"
```

Parse Tree 2:
```
S
/ \
NP VP
| |
"the" V
| |
N NP
| |
"cat" |
NP
/ \
"the" N
| |
"mouse"
|
PP
|
P
|
NP
/ \
"with" N
| |
"a stick"
```

As we can see, the sentence "the cat chased the mouse with a stick" can be parsed in two different ways based on the grammar rules. Therefore, this grammar is ambiguous.

Conclusion:
Based on the explanation and example above, we can conclude that a grammar that produces more than one parse tree for a sentence is called ambiguous.