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Practical LLM Systems Guide

用語集 · 15 terms

Glossary

Token

A piece of text that the model processes as a unit.

Engineer translation: The atomic unit of model input/output. Billing, context limits, and latency all operate on token counts.

First appears: Lesson 00a · Related: Tokenization, Context window

Logit

A raw score for each possible next token. Not a probability.

Engineer translation: Unnormalized output of the model's final layer, before softmax.

First appears: Lesson 01 · Related: Softmax, Decoding

Softmax

Converts raw scores into probabilities that sum to 1.

Engineer translation: exp(z_i) / sum(exp(z_j)). Applied to logits.

First appears: Lesson 05 · Related: Logit, Temperature

Temperature

Reshapes probability distribution before selection. Lower = predictable, higher = varied.

Engineer translation: Divides logits before softmax. T approaches 0 approaches greedy; most APIs treat 0 as greedy mode. Does not improve factuality.

First appears: Lesson 05 · Related: Softmax, Top-k, Top-p

Context window

Maximum tokens the model can process in one request.

Engineer translation: Fixed by architecture and training. Everything for one inference must fit.

First appears: Lesson 00c · Related: Token, KV cache

KV cache

Temporary store of computed attention state to avoid reprocessing.

Engineer translation: Each layer retains key/value tensors for prior tokens. Grows with sequence length.

First appears: Lesson 01 · Related: Context window, Attention

Embedding

A vector representing meaning. Similar meanings are nearby.

Engineer translation: Learned mapping from input to high-dimensional vector space.

First appears: Lesson 00b · Related: Cosine similarity, RAG

Cosine similarity

Directional similarity between two vectors, ignoring magnitude. Range -1 to 1.

Engineer translation: dot(A,B) / (||A|| * ||B||). Not a probability of correctness.

First appears: Lesson 04 · Related: Embedding, Vector search

Hallucination

Fluent, confident output that is factually wrong or unsupported.

Engineer translation: Catch-all for factual error, unsupported claim, unfaithfulness. Use precise metrics.

First appears: Lesson 06 · Related: RAG, Citation

RAG

Finding external evidence and adding it to context before generating.

Engineer translation: Pipeline: ingest, embed, retrieve, authorize, generate, verify. Does not modify weights.

First appears: Lesson 07 · Related: Embedding, Hallucination

Prompt injection

Attack where untrusted text overrides trusted instructions.

Engineer translation: OWASP LLM01:2025. Model cannot distinguish instructions from data.

First appears: Lesson 11 · Related: Tool calling, Authorization

Fine-tuning

Permanently adjusting model weights to change behavior.

Engineer translation: Continued training on curated data. Teaches patterns, not facts.

First appears: Lesson 08 · Related: Pretraining, RAG

Evaluation

Measuring whether output meets your quality bar.

Engineer translation: Versioned dataset + deterministic checks + model graders + human review.

First appears: Lesson 09 · Related: Testing, Model grader

Agent

A system where the model decides what to do next.

Engineer translation: Loop with explicit state machine, tool policies, budgets, stop conditions.

First appears: Lesson 12 · Related: Tool calling, Bounded autonomy

Attention

Mechanism for looking at relationships between all tokens simultaneously.

Engineer translation: query × key → weight; weight × value → output. Learned, dynamic, quadratic scaling.

First appears: Lesson 00b · Related: Transformer, Embedding